There is a section on these forums "Beginner's Starting Point" that includes specific advice on how to cycle a tank. The purpose of cycling a tank is to develop a large volume of "good" bacteria that do their job of keeping the amount of ammonia and nitrites down (both deadly to fish in higher quantities). Unlike wild fish where their river or stream would wash away ammonia from their droppings and food decay, fish tanks are extremely suceptible to getting an overload of those nasty fish and food byproducts. Cycling involves treating a fishless tank (ideally) with doses of ammonia, and as time goes on the bacteria that starts to grow in the gravel and in your filter media converts that ammonia to nitrites (still a hazard) and as time continues, new bacteria form that convert the Nitrites to Nitrates - you can buy a kit (the best one is from API) to measure the levels of all these things both during cycling and just periodically to keep your water in check. Nitrates are not particularly harmful but indicate you have a dirty tank and it's time to do a water change. People have strong opinions about water changes from doing only 20% every week or two to doing 70-80% weekly. A heavily loaded tank is going to need more frequent water changes. Personally I quite religiously do a 70-80% water change weekly. Since I'm disabled and it takes me about 4 hrs a tank to complete one I have an assitant I pay to do them for me - I do all the water testing and make recommendations, and sometimes I do get stuck doing them myself - but it's kind of a cool way to get to know your fish. Cycling a tank can take 30-60 days. I have found that if I add a bottle of Tetra's safe start with bacteria than my tank cycles almost immediately because I've added all this free bacteria instead of waiting for them to grow. My fish stores don't even bother to discuss cycling - just add the bacteria and go with it but I still think you should test your water for a period of time and as you test you'll see a rise in Ammonia (because you would have added ammonia to the tank) and then the ammonia values will decrease and you'll start seeing a slow increase in nitrites, and over time, ammonia will read zero within 24 hours after adding, as will nitrites and all will be converted to nitrates. It's pretty cool to experience. I still recommend adding a bottle of bacteria to your tank before getting fish especially if you are getting more than a couple fish - in the beginning the amount of your bacteria is low and too many fish at once will overload the tank. Adding a bottle of bacteria gives you a huge safely margin and you may be able to start with 4-6 fish depending on size and type (goldfish for example being pretty filthy fish compared to other species).
As far as plants - there is a whole forum devoted to plants. Get the simple ones like swords first and some floating plants. I don't have caves big enough for my Dojo's but they love to nap in the thick mesh of plants at the top of the tank. Just be aware you may be cleaning out your filter intake tube constantly with some floating plants - I have to clean mine out 3-4 times a day because of the DoJo's but the same plant in my Gourami's tank stays in place and rarely clogs anything. I personally hate frogbit and any plant that releases a gazillion nastly messy little round pods that end up covering my arms every time I have to fix something in the tank - I've got some kind of moss growing now that is marginally better behaved. About 2/3rd of my plants die within a week or get eaten (my Dojo's will uproot and eat almost any plant but they leave my swords alone. I always do a mix of live and artificial planting - the artificials have started to look so real (as long as you aren't buying gaudy colors and stuff).
My 50 (soon to be 100 gallon tank) has 6 Dojo Loaches in it that have grown to about 1 foot long. Amazing and playful and highly active they drive the poor Rainbow fish nutty (as they did before in my Gourami tank). At the size they are (and may not stop growing for a little while) they really need to be in a larger tank than a 50 gallon and they need to be by themselves so they don't drive the whole tank crazy. I am in LOVE with this species so it's worth it to invest in a much larger tank for them. Then I'm moving my Gourami in with my Rainbow fish (even though the Gourami's need a little softer water - I'm not providing that now) in to the 50 gallon tank and retiring my 29 gallon that housed the Gourami's or I may just keep all three and work on water softening for my Gourami's to extend their lives. I feel like I owe it to them. My rainbow fish do great in moderaterly hard/soft water as I have but I have to adjust every ounce of water I add to a PH of 7 because our city water, with it's PH of 9.4 is too high for ANY fish to have a healthy life in. Problem is, I'm paying a small fortune monthly for weekly water changes and I pay by the tank so financially I don't want to end up with a large number of tanks - it's just too expensive. So I may have to re-home my Gourami. There are just a few but I love them so.
Finally your betta. Also known as Siamese fighting fish until recently were always kept by themselves in a small tank (some ridiculously small). Mine is in a single 5 gallon tank and he is quite friendly to me - I shake my hair and he will shake his fins - too cute. Your angel fish and any other fish with large fins could be seen as a threat to your betta. I usually recommend a 5 gallon tank for the betta by himself - possibly with a small school of very small and fast fish. They aren't generally threatened by them and couldn't catch them anyway. In your case it sounds like the more aggressive Angel fish (not a species I've ever kept) attacked your betta but their fins would have been a big attraction and threat to the fighting fish. Betta's also get along with small corydora's and most small Raspdora's. None are perceived as being a threat.
As far as keeping really small fish in a 50 gallon tank - I do not recommend it. I cannot count how many have accidently gotten sucked up during water changes or just slowly disappeared - they are just too small to work with in a 50 gallon tank. I would look at fish that are 2-3" in size and perhaps a couple of pleco on the ground. The yellow or albino Bristlenose pleco is a cool looking fish BUT they too start out very small and take 2 yrs to reach their mature 5 inch size but they are great little scavengers of food in the tank as well as algae - although I have to feed mine algae tablets because I have very little algae. Just realize that a betta is a very aggressive fighting fish so any other fish with him is a potential threat, but they are just so slow due to their long fins that they rarely can catch small fast fish and aren't interested in ground living fish. I would keep him away from the angel fish which have their own level of aggression and long fins that a betta might find as threatening. Nobody should be sticking fish in your aquarium unless they know what the hell they are doing. Different fish require different PH, water hardness, or they may not get along etc. You have to do your research in order to carefully pick out a fish that will work in your tank and that is your decision, not somebody elses. They've now set you up for keeping him in another tank to himself while you search for companions or let him live alone with the snails,
Make sure any lighting you buy suits the plants your have or can be adjusted. I have my tanks covered with a bar of LED lights that can be adjusted to different strength - the best can cost a lot. But just a moderately lit bar of LED's you can get for about $50-60. You need this if you're going to do live plants or if you ever want to see your fish.
Hope that's enough info but get used to doing some of your own research so nobody has to spoon-feed you any basic information because there are plenty of good posts on this forum about the subjects you are mentioning.
