Mark,
As you can tell from my user name I'm a Bmxer. I started racing in 1984 & last raced in 2009. I didn't race all of those years but most of them. I still go ride the track & dirt jump my Standard 125r 20 inch & I turn 38 this year.
But enough about me lets get to you.
Do you know how to fall? Did you play sports while growing up say football , baseball etc,etc? Sport that made you dive for the turf or get knocked over. If so were you injury prone or did you just get a bump or bruise & bounce back up? Chances are if you were a good faller then you will still be a good at falling & not getting hurt. May sound funny but I've crashed 1000nds of times & give my all at any sport I take part in & because I fall well I very rarely get more than some road rash , bruises & maybe some stiffness.
Protection. Helmet , full face for sure. You may want to think of some other items as well. The last few years I raced I got sick of getting road rash when I crashed so I went & got some 661 mountain bike elbow/forearm & knee/shin pads. Man what a difference it was great not having to got to work the next day with those type of injuries. Lastly , Shoulder pads motocross/d.h. style. I never used them but many people do & over the last few year the have gotten way less cumbersome. Now maybe some people will think/say this is overkill but who cares if it helps you ride with confidence.
The Racing. Dude forget about everyone else on the track & just ride your race. If you get last, "who cares" your doing something you have always wanted to do. Ride within yourself & as your skills improve you'll get faster. Just have fun & trust yourself. Practice til you feel a little tired then call it a day & never , never say "just one more lap,jump,speed jump" because in bmx that's just bad karma.
Crashing. Eject,eject,eject. As soon as you realize your going down & can't save it, try & get ready to hit the ground & to do that you have to let go of the bars. Crashing is a art & those who can do it well very rarely get badly injured. You may have this skill from your youth & not even know it.
I hope some of this helps.
Jason