I have a troy-bilt 2550 psi and just use one time I put oil and gas but now It won't want start the pull cord is hard what i can do

When starting make sure you pull the handle to spray so it relieves the pressure from the pump an engine

Troy Bilt 2550 PSI Operator Manual

There is no need to prime. But you do need to use the black spray nozzle to get soap to siphon out.
if that doesn't solve the problem let me know

Try this link. I think you'll find what you're looking for.
http://www.troybilt-gen-pw.com/owners_manual/

Best oil for small engine for most moderate climates is 30wt non-detergent. The pump can use mineral oil (air compressor oil ) sold at most auto stores and lumber outlets.

I have the same model...... it should be just underneath the unit.... it will look like a square head recessed. I used the ratchet set extension and it fit just right. I literally had to tilt the darn thing upside down to find it. Then once it was loosen a bit, I put the machine upright on the wheels again, and slip a plastic pan underneath as I finished unscrewing the oil plug, Out came the black goo.

1st I would drain the gas and put new in. It loses octane over time. Then try new spark plug. Also you may need a new diaphram in the carburetor. They are cheap and sell them at most hardware stores.

What fluid and how much is required in the Troy-Bilt 2550 power washer pump?

remove the spark plug.  And pull the cord again.  if the carb float is stuck down or you overfilled the oil you could be hydro-locking the engine.  Check.