The first time I snogged a guy it just happened ... it was strange for a while. It's awesome fun once you're both good at it but it can start a bit messy ... just go with the flow and try not to be self concious ... you'll get better with practice.
If you go for a full make-out ... don't let him put his hands (or anything else) anywhere you don't want them. If you feel uncomfortable, pull away, tell him to stop or just casually move his hand away from where ever it is. Don't be scared to stop.

Enjoy!!
JT