Is it a 20 gallon long tank or 20 gallon tall tank?
What kind of fish did you get?
What kind do you want to get? Some are schooling/shoaling fish and need groups.
If you can, a 40G would be a great size for a community tank. Also it really depends on chich fish you are interested in which size would be best.
Read the article in my signature, it will be a great help. There are links for fish-in cycling.
When you do get the new tank, a 5G bucket and a clean rinsed pitcher and a strainer (dollar store is a cheap source for tank helps). Remove the decor pieces into the bucket nearly half full of freshly treated /dechlorinated same temperature water.
Make sure to unplug heater and filter, light.
Add the water from the tank to the bucket so the water will be similar.
Capture the fish gently with a net. Try not to chase them around to stress or exhaustion.
Move the water that is the cleanest from the top of the tank, leave a couple inches or more if it is not to clean looking. Use a plastic container to put the substrate into the strainer and gently shake to make sure it is free of poop and food.
Put it into the new tank. Water plants in the yard with the left over water from the old tank.
Use the water from the bucket with fish and decor - scoop to around half into new tank.
Add back freshly treated water to the bucket and then put half into the tank, add all the decor back into the tank and fish, and then fill the rest of the way with freshly treated water.
The reason for doing it gradually is to get the tank water of the old water and the new water to be similar and reduce stress on the fish.
Hook up the filter to the new tank as well as any air bubblers, heater.
**HEATER**************Make sure to unplug, especially the heater first before reducing the tank water. Make sure it is set aside carefully as it could be hot. Make sure to have it in the water per heater instructions before plugging it back in.
Check all items to make sure they are running as they should.