Gender...

Where's the best place to find out the gender of my BP? I bought it from PetCo and they didn't say.

The best place- A vet that treats BPs, or a breeder.

From there, you will just have to get to know people you run into at the pet stores or word of mouth, then ask them who can do it.
Use judicious sense though.

There are two ways to sex the snakes: popping and probing.

Probing with the proper tools is the most accurate, reliable way.

Popping is somewhat controversial, unless you are experienced and know what you are doing, fine. The problem is, someone with no experience may have seen it done on you tube and now wants to practice on your snake and can harm him.

Now probing can be easily learned on YouTube and the tools bought cheaply enough on Amazon.

There may even be a "Sticky" thread on probing here, or do a search.

Pop or probe, I prefer to pop because a visual leaved no question.
If probing is not done correctly you can damage the female too.

There's lots of videos on youtube that can show you how to properly sex your snakes, whether you want to try the popping or probing technique. Probing will require you to do get a kit of specialized tools and isn't as accurate as popping, but it's can be easier to do and requires less technical skill. If you're unsure if you can do either yourself, do as Reinz suggested and find a person near you who has some experience with it and can either do it for you, or show you how to do it yourself while they supervise. Once you've learned proper technique, it gets easier with practice.