Hi Becky - Difficult question! Baga, Calangute and Candolim are next to each other along the beach - they all merge into each other. Baga tends to be younger and more clubs and night spots - the beach is very busy these days. Calangute is busy, busy, busy with loads of shops, bars, restaurants and things going on. Candolim has plenty of bars, restaurants and some of the larger hotels that the "package" holidays use.
I prefer Candolim but you will need to read the reviews and do a bit of homework to see what you like the sound of. They're all close to each other and taxis aren't expensive so it's easy to get about..
When you've done some homework, pitch in another question - there are some very knowledgeable and helpful people in this forum!