Is it a 20 gallon long, or 20 gallon high? Here is a quick overview I wrote a while back:
Alright, I'm by no means a planted tank expert but I will help you out best I can.
To have a successful planted tank you should have three things:
Good light
Fertilizers
co2 or a carbon source
For light, I would get a T5HO (high output) light fixture. If you do a two bulb fixture, with good quality bulbs that will put you at medium-high light. With a four bulb fixture, that will put you at high light and you will be able to grow pretty much anything your heart may desire. If you have never kept planted tanks before, I would start with a dual fixture. If you want to down the road, you could add another.
Fertilizers come in liquid, dry, and tablet form. Liquid ferts are easy to use, but not very cost effective after a while. They also aren't very customizable. A lot of people, like me, use dry ferts. You pretty much buy some, they are kinda like powder, and mix a certain amount in water and either dose once a week (EI dosing) or everyday (PPS-PRO dosing). EI gives you more ferts than you need, so you have to do 50% water changes every week. PPS-PRO is more customizable, and cheaper in the long run as you dose everyday, exactly what the plants need. Ferts also come in tab form that you push under the substrate. These are used for heavy root feeders like swords, crypts and ferns.
Co2. There is a minimal amount of co2 in the water column, so in order to boost plant growth (and some plants won't even grow without it) you should add co2, or at least a liquid carbon source like glute (metricide 14). Pressurized co2 works better, but getting started can be expensive. Liquid carbon is cheaper, you need to dose it everyday, and can be more expensive over the long run, but a lot of people chose it over pressurized co2.
Lastly, you should have a good substrate like Eco complete, aqua soil, or even organic potting mix capped with sand. You can use plain gravel, but the plants will grow better in a plant specific substrate.
Hopefully this helps, ask if you have questions
ETA- For dwarf hairgrass, since it is a bit high maintenance, you will want high light, co2, and ferts.