I assume you mean a skatepark?
Start by just sessioning small obstacles to get the feel of the bike. The first time on a BMX will feel very strange, and probably quite wobbly if coming over from a MTB. If its an indoor park (wood) the chances are your bike will also feel very slidy, so try and scope out how much grip you have at the start.
I’d say for a first time the main things you want to concentrate on are the feel of the bike, bunny hopping, and dropping in from flat banks first, then quarter pipes with coping next. Look at what other people are doing, and ask for tips – people are usually very helpful.
Also, take a few minutes to look at what other lines people are taking around the Park and follow suit / take note. you’ll quickly see what groups are sessioning what area / feature, and see what lines they are taking to get there.
The main thing is to just chill out, and take your turn when it comes. If it’s busy you can honestly do more sitting around waiting and chatting than actually riding, its just the way things go.
Pretty much the number one rule is to always wait until its clear to go, so as to ensure you don’t take someone else out or someone takes you out. It WILL happen eventually, but by being vigilant and courteous you can try and make sure it doesn’t happen often!
Skateparks are a mine field of etiquette, but they are a whole heap of fun at the same time! Enjoy!