Welcome to the forum
Welcome to the wonderful world of fishkeeping
are you planning on replacing the 5gal altogether and moving your betta into the new one? Or keeping the 5gal with the betta in it?
There are lots of things to consider:
1) do you know where you'll be putting the new tank? If so how much space do you have as that will determine the size of tank you can have?
2) are you planning on keeping the 5gal as I said above?
3) if not, are you wanting the larger tank for your betta, or do you want to keep more fish? This can have a big influence on the equipment you'll need.
4) if you're not keeping the 5gal, do you have a list of your current equipment? If we know that we can see whether any of your existing equipment can be used in the new tank, therefore saving money.
5) do you have any idea what you'd like the tank to look like? I know you said you want plants but do you know what else?
Have a look around the forum. There are lots of photos of other members' tanks about. Have a look in the competitions section, lots of inspiration there. Have a look here too www.fishforums.net/index.php?/topic/277264-beginners-resource-centre/ for loads of information on where to start. Once you have an idea of what you want, we can give more specific advice rather than general comments. The most important thing is research. It's much better to spend some time finding what you want, than diving straight in. If you've found the forum you're already on you way
Almost everything I know I learnt on this forum. You'll find everything you need