I've seen a few confused noobies at fields before and to prevent you from wandering around like a brain damaged puppy here is a quick rundown.
Before heading out to the field make sure they allow "walk-ons" some fields are by appointment only, some have designated days for walk-ons, and some allow walk-ons all the time. You don't want to show up and find the field locked up because you didn't call ahead. By the same token you don't want to get there and find that little Timmy's birthday party has the field booked and you aren't welcome to join. When you call also use the opportunity to see if they allow you to bring your own paint. They will probably just laugh but it never hurts to ask. Depending on your age you may need your mommy to sign a permission form. Try to hang back a bit to preserve your pride.
When you get to the field look for where you sign in. There will probably be a bored looking teenager manning a counter or if you are lucky a line up. You will need to sign a form agreeing not to sue if you die on the field and to provide a number to call to inform your next of kin. You will also have to pay the field fee and buy your paint here.
Your next step is to find a table or an unoccupied clearing to deposit your gear a "base camp" if you will. If you've arrived a bit late the regulars will have poached all the good spots take note for next time and look for some nice floor real estate. Load up your pods and gas up your marker.
Make sure you have a barrel condom/cover and that you are using it! Maybe your friends don't care but field operators and most players are dead serious about safety and if you don't want to piss anyone off or be booted from the field you should make safety your number one priority.
After loading up your paint and becoming one with marker safety you now need to chronograph your marker. Make sure you have your mask handy and look for the chronograph station. If you are wearing a handy four leaf clover there might be sign saying "chronograph station" but since that is rarely the case listen for the agglets shooting. No matter where you play there will be kids showing off how fast their markers can shoot and it is usually over the chronograph.
If you don't know how to use it just ask and if you don't want to admit that you are new just say you don't know how that particular kind of chronograph works. Everyone will still know you are new but you can pretend otherwise. The point of the chronograph is to make sure you are shooting under the field velocity limit usually 280fps if you don't know the field limit
ASK!!! If a ref finds you are shooting over the limit you could get the boot or even hurt someone. If you hurt someone you will likely get a permaboot and how embarrassing would that be? Be sure you know how to adjust your marker's velocity (for extra veteran points bring the proper allen key) put on your mask and fire over the chronograph to find your velocity and adjust up or down accordingly. When you get it shooting where you want it to put your barrel cover back on and head back to your floor clearing.
Some fields that cater largely to renters may not let you chronograph your own marker instead a staff member will do it for you. Also some fields will have a ref supervising the chronograph and will put a small ribbon or piece of tape on your marker saying that it is within field limits. Again if you are unsure just ask. Safety questions are always well received and people will think you some kind of paintball super god or at least a noobie with his head in the right place.
Prepare to be saluted for you are now about to rock.