Do you want to do a planted tank, or just silk/plastic ?
You can start by picking up a filter. One of those Whisper 10's should do it. They hang inside the tank and are very quiet.
If you plan on trying a planted tank, get a good substrate for planted tanks. Flourite or Echo-Complete..the Echo is pretty expensive, but I find it works wonderfully for my planted tanks.
A couple of fancy guppies would do ok in a 5 gallon...don't get any females though..lol..they breed like flies ! With a tank that small though, you would have to be careful about overloading. I would try 3 fancy guppies to start with...but do what you can to get the tank cycled before you add them...if you can...if not, make sure to do frequent water changes during the process.
Maybe, once the tank is cycled and if you do live plants...you may be able to add a couple of dwarf corys, to help keep the bottom cleaned up..but that would be about it I think for a 5 gallon.
With live plants you should be able to keep more fish in there than a tank without. As long as there is enough light for the plants to do well...figure about 2 watts per gallon, so that you don't have to do CO2...and some low light plants.
I have a 5 gallon that has one 10 watt compact flourescent bulb, and my plants seem to be doing very well..here's a pic..
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