Im sure you are leaving your little man in capable hands that you trust fully so have faith in yourself and your judgement.
Do leave numbers and contacts just in case so that you will feel safe in the fact you are contactable and vice versa.
Allow yourself two phone calls home to check in and stick to this or you will end up on the phone all night and your sitter will feel you dont think they are capable.
Leave the babysitter a teeshirt or something of yours so that if your baby becomes upset he will be able to smell you to settle.
It is a bit daunting leaving your baby for the first time but only if you let it. Go and enjoy your night out and yes think about your little man but dont let the worry take over your good time.