Shofars are blown much like trumpets..anyone who plays a similar instrument has a starting advantage. A short shofar is also easier to sound than a long one and uses less air.
You'll want a tight upper lip, and looser lower lip, then when you blow out, your lips will vibrate. Put the mouthpiece against your lips (not inside your mouth) and try to force air through as small a hole in your lips as possible. I put the shofar in the corner of my mouth, others use the center..it's up to you. I also use my fingers and wear ChapStick to help create a good seal.
The goal isn't to force as much air out as possible, so don't tire yourself trying to make a sound. It just takes practice.