I know its quite unfashoinable today to say it but I started out with a 18x10x10 tank and got a great deal of pleasure from the fish I had.
Suggestions:
1) Endler guppies (or failing that guppies)
2) Neon tetras or glowlights or other small characins.
3) Wolf will disagree with me but as a child I kept and bred zebra danios in such a tank. However it would be better to get a smaller species such as Danio Nigrofasciatus or Choprae if you can (both available at Wildwoods Middx) and don't have anymore than 6.
What you will have to do is change the water regulairly and make sure the filter is ok. If you are unable to get an external canister filter I would recommend an undergravel filter.
Plants- in my experience, fish are happier in a small tank if it is well planted. Unless your water is soft I recommend using rainwater if you want the plants to thirve.
If that's the only tank you can have, then you won't be able to raise any of the fry that you breed, because they will get eaten or cause problems with overcrowding.
A betta would work ok, and perhaps a group of corydoras or livebearers (depending on size).