Good luck with that. Vodka is a hard one to start out on. I hope you will be responsible and have a designated driver. Also, it's fine to get a little buzz going but don't drink so much that you lose control. You risk alcohol poisoning as well as participating in things that you normally would not do.
If you really want to get tipsy, I'd suggest wine coolers. If vodka is all you have, how about making screwdrivers with orange juice? At least that way, you'll get your vitamin C, too. If you have jello, you could substitute the vodka for the water and make jello shots. I use plastic condiment cups that you can buy in bulk at places like Sam's Club. They take a little while to set up but they are fun and you'll still get a buzz.
As for how much you need to drink to feel it, that's up to you and depends on your size as well as your metabolism. It shouldn't take much, though, a few shots. Be careful.