1. You might want to turn down your game volume lower then your voice over. ( -23 if using Vegas )
2. You seem to have a lot of background sound ( like bumping and stuff ) not sure what that is. Maybe get a new mic if your serious about commentary
3. Instead of saying "Umm" just pause.
4. Make a list of thing your going to talk about before so you don't have a dull spot.
5. Also, try talking louder, it hard to hear what your are saying.
6. Remember its okay to redo it a few times if you don't think its good. No one is perfect
Hope this helps