I echo what has been said above, if this is your first fish keeping venture - don't go for marine fish. It doesn't sound like your tank is meant for marine, just a small freshwater tank.

I've kept fish for years now and after years of Discus, we decided to get a nano tank and keep a couple of small marine fish and corals. It is a lot more complex then fresh water and cost us ALOT of money, for a tiny nano tank we must have spent a good £500 on all the bits and bobs you need.

My best suggestion is to go ahead and buy a heater, then you can keep some nice colourful tropical fish. 1st thing - you MUST let the filter cycle first. This means having your tank running with nothing in it - no fish at all - for about a month or so.
Read up about it on the net, plenty of info about starting a tank.

Once you've done that, gradually add some fish. You won't be able to keep many in that little tank. Maybe some guppies or neons, comething bright