Moss balls are insanely easy. You can get them on ebay ( I got 5 for a fiver ) . They are a type of algae, but it doesn't spread anywhere, it just stays in a fuzzy ball. You just chuck them in and leave them alone to roll about on the bottom . They do tend to float at first but don't worry, they soon soak up te water and sink to the bottom. You may want to give them an occasional whoosh around in a bucket of tankwater when you do water chnges to get any bits off that may have stuck on them but that's about it.
Java Fern is a low light plant that's pretty easy to grow. Again, doesn't take too much maintenence. It doesn't need to be rooted in the bottom, you tie this plant to rocks or wood with some fine fishing line. In the past I've found it's more easily attatched to a smooth piece of bogwood ( make sure it IS smooth so your betta doesn't tear his fins on it ) . Just tie in on at the base above the roots and leave it. The roots just dangle down.
Java moss, like java fern, is meant to be tied to rocks or wood. If you tie it to wood, lay the strands of moss along the wood ( take the wood out to do this ) and wrap the fishing line around firmly before tying it tightly.
Elodia is another easy plant that will grow fine in basic light. It does grow tall and will reach the surface and start to go along it. When that happens, just pinch or cut it off about 3-5 cm under the water's surface, and it'll re grow until you do the same again. It just needs a regular trim once it's growing well.
Cabomba is often sold in mixed bunches with Elodia and is another good plant to grow. Bettas love it . Both this and elodia are usually sold as stems ( no roots on them ) but this is fine as roots will grow on most of them, just plant the stems in the gravel or sand , in groups of about 3 stems each, deep enough so they won't float up. Same as Elodia, just pinch/cut off the tips when the plants get too long.