Chat to everyone, get involved in your games, communicate and above all else Enjoy yourself!
Also as above, Marker, Bottle, Hopper, Clothing, Mask, Spare pair of Socks/Shoes can be a must depending on where you play.
Don't be afraid to ask for help if you need it or to know where things are like the filling station or how to fill etc.
Any problems with your marker/kit aswell, don't be afraid to ask someone to help if you are unsure whats wrong.
Also try not to overshoot, it's easily done but in my opinion ruins a Walk on, as it's meant to be more relaxed and fun.
Oh yeah and a Battery (If your marker/hopper uses one)
Check your kit before you leave, last thing you want is to turn up missing something you wanted.
EDIT: As Bolter said a Towel or two is a good idea aswell, and maybe a Microfiber or two for your mask.
Hope this helps and that you enjoy it.
Thanks
James T