installed cowl on plane for the first time!!!
 

Installed the cowl on my plane , plane had been trimmed out earlier took off next minute plane started a steep climb didn't understand why and still don't has any body had this happen to them before?

Didn't something shift aft?
Other than that you're more aerodynamic with the cowl on. Reset trim at half throttle or so and see what happens

Who knows what kind of plane you have but I don't know of any where an installed cowl would push the nose up that much. Maybe a slight difference but nothing dramatic. I would check the elevator surfaces and linkages and as mentioned the CG.

You would be amazed what can cause a trim change. Last race season I lost a stripe of covering off the bottom of my wing. This left a bare wood area about 7 inches long and 3 inches wide. The airplane took half of my available up trim. After the heat I covered the section with packing tape. Next heat I had to take the up trim out. A cowl will certainly cause a difference in trim. Then again the OP doesn't even state what airplane let alone post a picture with cowl off.