I have a reactive dog and have been doing rescue a long time, so I'll speak from experience...
Leaving the leash on actually makes things a LOT worse. Dogs have totally different reactions to meeting other dogs on leash than they do off. Some will defend "their space" (taking the range the leash reaches to be theirs) and others will feel like they can't get away if scared and will react because of that.
They can also be very dangerous in a dog park situation, and dogs can easily get tied up in knots, hurt, or worse. A lot of dog parks prohibit leashed dogs inside for this reason.
If you're really unsure how your dog will be, walk him/her outside the fence on leash for a while and see how they react. If they seem really unsure or actually react to the other dogs inside (growling, lunging, barking...), don't push your luck. And if any of the dogs inside freak out at your dog's presence at the fence - play it safe and do not go in.
I find it helps a lot to scope out the other dogs already inside when you arrive - particularly their sizes/ages and play styles and how closely their owners are paying attention to them. (Loads of problems occur at dog parks when clueless owners don't pay attention to their dogs or allow inappropriate behavior). I always watch for a few minutes before going in with my dog to make sure the dogs already there seem friendly and appropriate in the way they're playing and interacting with each other.
The first time out, you might want to steer clear of dogs that are a lot bigger than yours or who have really boisterous or "forward" play styles.
It's also really helpful if your dog has a good recall and can be called away (even with distractions) if it looks like things are getting a little out of hand.
In general, if you're dog isn't well socialized with other dogs already, a dog park is not a great place for them to start. And if they have a bad experience there, you now have a new (and hard to fix) issue to deal with.
Trust what you know about your dog and when in doubt, err on the side of caution. Remember that FW has lots of nice dog walks you can use as well. And while it's great if your dog can do well at the dog park, they definitely aren't for everyone.
Of our 3 dogs, I know that 1 isn't great meeting strange dogs. He isn't out to hurt anyone, but his greetings are obnoxious and can come across really badly. I don't think it's fair to subject anyone else's dogs to that (or let him practice bad, self-rewarding behavior) so he and I will go to the dog walk together while DH takes "the girls" to the dog park to play.