here's a short one.
the radar needs time to detect enemies at long range. for BVR, ask your AWACS for the heading for the enemies, and then use your radar. if you fly the F-15, the enemies will be displayed as rectangels and friendlies will be displayed as a circle. to lock a target, slide the two vertical bars over the target you want to lock, and press TAB (by default). to change the range, use 0 and + or + and \. (don't remember clearly) If you fly Su-27, Su-33 or MiG-29, the enemies are shown as triangles. Friendlies are displayed as a circle. You will notice a line. The line points in the direction the other plane is moving in. If there is a friendly AWACS flying out there, it will send its own picture on your radar, so you can be passive. This lets you also to find your targets more easily.
as for dogfight, it's almost impossible to use your own radar. If you fly Su's or MiG's and a friendly AWACS is flying, you can use the AWACS, cuz the AWACS sends its own radar picture to you.
hope that helped you somehow. i may have forgot something.