Firstly, I'll advise learning to post before learning to sit. The reason is simple - posting gives the horse's back a break while you're learning to sit for short bursts. Honestly, the trot shouldn't be sat unless the horse is relaxed and rounding his back up into your seat. Trying to sit the trot on a tense and inverted horse is only going to set both of you up for discomfort at best. An inverted horse is going to be rougher and harder to sit, and that difficulty can make a beginner bounce worse, contributing to further tension and inversion, etc. etc.
When the horse is rounding and relaxed at a posting trot, try sitting. Most important is to relax all of your joints. Your toe-joints, your ankles, knees, hips, waist, back, shoulders, neck... everything needs to be relaxed to absorb the motion. Any tension will contribute to bounce. Go through the mental checklist, starting with your toes and moving up to your head.
Another mental thing to think about is moving your hips with the horse's hips. Is your left hip moving down and forward when his left hip is? You want to "trot with him," not fight his natural motion. Finally, remember to keep breathing! If you hold your breath, you WILL tense up, and probably get a side-ache. If you do get a side-ache as you're practicing, come down to a walk, breath, and press on the ache with your hand - it'll go away pretty quickly.
Good luck! It may take a while to get the hang of riding the trot well, but that's perfectly natural. Just keep at it, and enjoy the ride!