1. If you see a scout, aim for the feet as soldier.
2. As a scout, jump on peoples heads and hit them with the bat. It works suprisingly well on heavies.
3. ALWAYS HEAL HEAVIES! If there is a medic healing the heavy, then heal the medic and when he dies heal the heavy. Also, pay attention to whats going on, not just your heal target!
4. As a spy, do not come out of your base disguised as an enemy, (eg. coming out of red base as a blue demoman) its super obvious and snipers will shoot you. Avoid pyros.
5. Don't be an offensive engineer.
I guess this will help?