Good luck with this. serbianschool.com is an easy (and free) online introduction to the language. Is is based on the Latin script, so that makes it even easier, and their are interactive excercises in each lesson. It's great. The only thing is the "animated" vocabulary sections - they are too quick to catch, but you can get most useful "survival " vocabulary at http://www.101languages.net/serbian/ which also gives you basic background.
A good tool for learning vocabulary is Interlex at http://www.vocab.co.uk/ . There is loads of Serbian lists in the Community Exchange - food, trave, etc. The only thing is, they are in Cyrillic, and some are a bit technical (the Body list had a broken arm and a heart attack in it ;-). The best thing is to make up your own lists, start with the multiple choice item into English and end up with the write-in into Serbian. It's like a game.