absolutely run what you brung. My first match was shot with a carry handle DPMS, a walther p99, and a pistol grip 500. I did terribly but I had a safe and good time, and it got me hooked. I got to the point where I was running 100% full custom later but that was when I was trying to break into the national level. life gets in the way. Anyway, my actual advice:
shoot slowly and make hits
be safe
watch your muzzles
watch your thumb loading that 870 or you will rip it to pieces. Ask me how I know
don't spend a dime on anything but ammo before the match
Talk to established shooters
talk to the shooters who shoot like you would like to. they will let you try/borrow gear and explain how they like their stuff set up
don't wear a war belt with a knife on it. you don't need a knife and will look like a tool.
otherwise go have fun!