For starters you need to look at as many tanks as possible to get an idea of what you like. Here is a link that has tons to look at...
Your Tanks - Browsing.
If you think you want to go with a planted tank I would suggest spending alittle more and getting a commercial plant substrate. Eco-complete is the one I use but there are many brands available. The reason you want a commercial plant substrate is because it has a high CEC, which means the substrate can absorb and hold nutrients from the water and detris and keep them available for plant use.
If you get a "kit" the stock light that comes with them are very low so you will be quite limited to what you can grow. You might want to consider buying the tank, then buying a better light fixture, like a 2 bulb T5HO fixture (which will allow you to grow alot of plants and not limit you like stock lighting will), then you'd need a filter and heater.
If you go with a kit and stock lighting then you will have to limit your plant choices to Anubia's, Java Fern, and Crypts. You might be able to grow water sprite or wisteria but that is a maybe. If you go with better lighting there are all kinds of easy plants you can use. Besides the ones listed above you can use Swords, some Rotala's, some Ludwiga, Bacopa, Vals, Dwarf Sag, Temple plant, Tiger Lotus, and many others.
For a 20g easy fish would be, small tetras such as Cardinals or Rummy nose, Rasbora's, small cory cats such as Panda's, Honey Gourami's, and a few others.