I have an 04 CRF250R, with a new plug, fresh gas, and a fresh top end( good compression), but it won't start, any ideas?

Not to sure without seeing or hearing it being pulled over. Compression? Very likely needs some carb work/kit.

dump all the old gas out, put in fresh stuff, sometimes you gotta change the spark plugs as well.
cheers bucky

Probably dead gas from sitting too long. Dump it out & put in fresh gas.Could have moisture in it. Pull out spark plug & crank it over several times after putting gas & let it sit a while to dry out or blow it out with compressed air. It should go.

There's a website that has all this information about all this and manuals everything totally free there are many actually but I personally like few ..
let me know if u need help
thanks

May be low engine compression. Replace gaskets, piston & cylinder.

Fuel pressure for your car should be 50/60 psi you probably need a new fuel pump.

My first thought is low compression... which leads to a few questions...
What fuel to oil ratio have you used?
What do you mean by 'fresh top end'? To me a fresh top end includes piston and rings and cylinder bore, gaskets and crank seals. Anything less on a 2-stroke is an incomplete job and usually leads to premature failure.
Have you followed break-in procedure on your fresh top end? Double check the head bolt torque.

Do a compression check first then go from there.

yeah something you wanna check next, compression. if you can get your hands on a compression tester.