If someone is planting or difussing the bomb, cover their back. Be a team player. Move forward in a group by leap frogging each other as you find a place to cover. Be ready to fallback and not be in the way. Don't worry about saving ammo, you will be dead otherwise.
Some guns can shoot through objects.
Also, if you crouch and sneak around a corner, your leg will be seen before you can see the enemy. The learning curve to CS is like no other game I have played. It will take time to learn.