Do you know what kind of filter it is and what size tank it's rated for? Usually you want at least the double amount of filtration for your size tank. Some filters are better than others too.
How are you planning to cycle the tank? Fish-in or fishless? This guide might help you decide:
Guide to Starting a Freshwater Aquarium - Aquarium Advice
African Dwarf Frogs are cute but social creatures and can be a bit hard to care for especially in a community tank. First, they shoot up to the surface for air and they aren't the best swimmers so a 20 gal tank may be too high for them (I'm assuming it's a 20 high and not a 20 long), plus the betta may not like them shooting up to the surface much and it's possible the frog will bang into the betta at some point. ADFs are blind so they often need to be target fed in order for them to get food. They are also social, so you'd want at least 3. It's possible they'll swallow small gravel bits and become compacted so the substrate choice is important. They really do better in their own setup, but people have kept them in community tanks. I'll let others say more on that.
As for the betta, again it's hit and miss in community tanks. Most bettas are aggressive by nature and don't tolerate tank mates well, particularly those with long fins like guppies. It wiill depend on the betta, really. You can try it, but have a back-up plan for the betta if it doesn't work out.
Tank mates will depend on whether you have the betta and/or frogs. You'll have more options if you decide not to have one or either of those. A 20 gal is a good size tank though so you do have options, almost too many to name really. What substrate are you going for (sand or gravel)?
You could do something like:
--1 betta (if it works out) for your centerpiece fish OR a dwarf gourami (not both)
--a school of 6-10 danios, tetras or rasboras. Most types would work. Harlequin rasbora, rummynose tetra, neon tetra are just some examples.
--if you forgo the frogs and have sand or rounded gravel as substrate you could do a group of 6 corys for the bottom; panda, dwarf or pygmy are some good options.
You might want to take some trips to local fish stores near you, write down what you like, research the fish and ask here so we can help with stocking.