whats the best filter?
In a tank your size, the filter is only really going to pull out debris. The chemicals, like ammonia, are still going to build up and require you to change the water regularly. Get an ammonia testing kit and check regularly. When you need to change water get some in a clean bucket and clean it with a chlorine/chloramine remover (tap water conditioner), then let the water sit overnight. The next day you can change out the water and clean out your filter.
whats the best depth of the water?
doesn't really matter as long as they can very easily get out. A general rule is that the more water the more stable, but also the more you have to change on a regular basis.FBT's don't like water as much as pet stores tell you. I have 20% water, 30% transition and 50% dry land.
should i provide heat?
This depends on you conditions. They should be between 70F and 80F heat as you need to maintain. I have central air and heat and just place a towel over the top when the tank gets too cold, generally in the summer.
what are the best plants?
right now i have pothos
Plants are just going to be for them to hide. They are carnivorous and won't eat the plants so you don't have to worry about poisinous plants. Plant what is easy to maintain for the conditions you have in your tank.
most people only feed their fbt crickets. If you are going to do this make sure to dust the crickets with vitamins so that they get all of the nutrients they need. If you are up to the challenge and have the space you can raise your own crickets and gutload them. Or you can but inexpensive wingless and flightless fruitfly kits and always have a fresh supply of good food on hand.
Good luck and enjoy your little ones