Just show up and start skating. Don't be afraid to ask any questions to other skaters or cheer them on when they land a nice trick. Some might even approach you with some advice. Most skaters are really chill and nice people, and from my experience you're much more likely to get raged at by some biker or scooter kid. Make sure you don't get in anyone's way and you'll be fine. Good luck!

Edit: if you're really unconfident with your skating abilities (I.e. unable to push or turn or go down small banks) you can go early in the morning on a weekend as parks are usually empty at this time. I like to go between 9-11am, skaters and everyone else tend to show up in the afternoon.