With a long thin tank like that you could get some tetras (the black skirts are particularly hardy I've found as mine survived all my beginners mistakes but not nearly as nice to look at), they're schooling fish so you'll need at very least three of the same species. Neons are popular too as a starter fish for small tanks. A warning on tetras, the ones who are named after fruit or candy (except the lemon ones) are usually injected with color and that's not good for the fish I'm told (my daughter was heartbroken when I told her she couldn't have the bubblegum tetras!).
Enders Liverbearers and Guppies would both work too and are wonderfully colorful but becareful not to mix male and females cause I'm told they can multiply like mad if you're not carefully. (Others can tell you more about these suckers I've never kept them personally though I'm thinking about them for my smaller tank).
You could have a single betta too (especially given your tank is long and narrow it would have lots of room to show off) but no other fish with brightly colored flowy fins (no guppies or enders) and no fin biters. Bettas and Cories in particular get along really well.
In a 6G you don't have a lot of choices for bottom feeders. DO NOT let the people at the petstore tell you you have room for a pleco. It looks small now but the suckers grow to 1 foot long! Full sized Cories also are small but they like to be in groups (as I've recently learned) and you're tank's pretty small (though being long would have more bottom room then most 6Gs). Pygmy cories are tiny and in a tank that size you could have a little grouping of them (they're sooo adorable), but they're harder to find.
Note: You definitly couldn't have all these fish at once. In a 6G you're looking at about 6 small fish, 5 if you go with the Betta. You'll definitly want to speak to the people at the fish store (and having had a ton of bad experiences with the Pet Store I go to a fish only place now: Big Al's usually) and let them know you're new to fish keeping and have a new uncycled tank.
While I'm sure this is all stretching your budget (fish keeping is expensive to start but really it's not bad once you're set up), the above test kits are really important. Aquarium Pharmasudicals and Nutrafin both sell freash water master test kits and this is by far the most cost effective way to go. You'll aslo need some sort of water conditioner and/or decholinizer, you can probably go with a smaller bottle now and get a larger one later. I also usually suggest a small bottle of Melafix which is basically fish medicine. I find it helps with stress introducing new fish to tanks too but it's definitly not nessesary (I didn't use it when I first started and all my fish were fine).
All these fish need a heater too. A cheap one can be had for a tank that size for under $15 and it's totally worth it.
All told you're looking at about $30.00 for the test kit, $15.00 for a heater, $10.00 for water conditioner/decholinizer $3.00 for food (buy small it lasts forever) and $10.00 for fish. It's not as bad as it sounds all this stuff will last you a long while. (My SO is always joking that the fish are the least valuable thing in the tank.)
Lastly you'd most definitly want to be slow about putting fish in the tank. We all wanna rush out and dump fish in our tanks... and we've all learned the hard way that you need to be patient!
I hope all this helps!
Edit: NOTE: Others will probably have a lot of information for you too. Be sure to read all the stickies on this thread there's TONS of valuable information there.
