Larger tanks are a bit pricier to set up, but after creating an ecosystem with good soil and plants, care can be as little as topping off the tank every couple days and cleaning the soil every 4 to 6 weeks. 20 gallons is ok. But after soil and ornaments, you will only have about 12 gallons of water. I find Bettas do much better with 30+ gal tanks. You will want to start with the Betta first. They all have different personalities. Then add schooling fish like tetra or some type of small fish that is not known for nipping fins. You will also need a heater, water conditioner, water test kit, quality food like Bug Bites. Bettas also like having a loft of some type near the surface. Its a huge learning curve and a fun hobby. But it can lead to aquarium addictions. I started with a 1 gallon betta tank and now i have 5 tanks totaling 120 gallons.
If you have Facebook, there's a aquarium co-op community that is awesome at answering questions! Beyond that, yes, 20 gallons should be easier. It's often said that the larger tank you can start with, the easier it is run it. That's due to the fact that water parameters change rapidly in smaller volumes of water, but usually take much longer at larger volumes. With the beta, make sure that it has some floating plants or still areas of the tank. They can get a bit beat up by too strong a filter, and appreciate the shelter.
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