I see that you're in WV, which means you're in District 5. http://www.amadistrict5.com/Default.aspx
If you plan on staying with your 85, you'll have 2 classes that you can run, the 85cc (<--NO BIG WHEELS ALLOWED) and supermini. However if you decide to get a 125 or 250f then your class options greatly increase.
Here's a PDF with the D5's rules, scroll down to page 8 as that's where the MX section begins.
http://www.district5ama.org/LinkClick.aspx?fileticket=T1K9Ow0z7+s=&tabid=384&mid=1032
I didn't see it saying anywhere what number you're supposed to use, however a lot of guys will use the last 3 numbers of their AMA number. Use this database to see if anyone in D5 already has it, if somebody does then you'll want to pick a new number. http://www.tracksideonlineresults.com/trackside/d5/search.asp
Here's a chart that shows number/background colors, if you run the youth 85 class it looks like you're supposed to have yellow backgrounds with black numbers. And the C class runs black backgrounds with white numbers. I personally would just run the black backgrounds with white letters and be done with it, I've never seen anyone get booted for having the wrong colors. 
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And don't forget what the flag colors mean. 
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