I'm still very much a beginner so I'm still learning.
When you say small, how small? I have a 15l planted cube with 12 Chili Rasbora and a handful of Cherry Shrimp and some snails. Some people will tell you a 15l tank is too small for any fish but Chili Rasbora are absolutely tiny so I can just about get away with it. I think the suggested minimum size to allow for a variety of fish is 30l but it's still limits what you can put in it. I also have an 85l tank which is more suitable for a range of nano fish.
Before considering anything, check the pH of the water which comes out of your taps. It'll make it much easier to maintain the tank if you only keep fish and invertebrates which like that pH level. Some fish like slightly acidic conditions others prefer more alkaline.
I'd suggest a planted tank as it helps keep the eco-system stable.
Buy a decent test kit - I use the NT Labs one. You'll need to test for pH but also ammonia, nitrites and nitrates. Ammonia and nitrites in the water will kill fish fairly quickly. You'll also need to test the water hardness (general and carbonate hardness) Learn about the Nitrogen cycle and how it applies in closed environments like fish tanks.
Be prepared to wait a long while before adding fish to allow the tank to cycle. It could be several weeks or possibly months. You have to be patient! There's stuff you can add to the tank to speed up the process though. Effectively, what you're trying to do is get colony of bacteria to develop which turn the ammonia into nitrites and then another colony of bacteria to develop to turn the nitrites into nitrates. At that point, the plants will consume the nitrates to help keep the environment stable. It's only once these various bacterial colonies have developed and they're cycling the ammonia through to nitrates that it's safe to add fish. These bacteria will live in the filter, on the plants and in the substrate you've used to anchor the plants. Add your fish in groups and wait at least a couple of weeks before adding any more - the tank eco-system needs to adjust to the new bio-load as the fish will produce ammonia via their waste so the bacterial colonies need to grow to cope with it.
As for fish, the pH and water hardness and the tank size will determine what you can put in it. My water is slightly alkaline (pH of between 7.5 and 8) and have the Chili Rasbora in the small tank and Harlequin Rasbora and Panda Corydoras in the larger one. Cherry Shrimp seem to be happy with my pH levels too. I don't think the snails really care about the parameters.
Expect to do more maintenance on the tank than you might expect - cleaning the glass, trimming plants and water changes/top ups.
Also be wary of the combination of the fish you choose. Some will eat shrimp and snails. Other's might even eat the other fish.
Check out Maidenhead Aquatics website which has a long list of fish and what water parameters they live best in.
www.fishkeeper.co.uk
There's a few Youtube channels worth watching
Fish Shop Matt - for no nonsense advice on fish and tank set ups
MD Fish Tanks and
MJ Aquascaping for inspirational stuff.