How do I aim at the xbox 360 in halo 3?

I used to love Halo and have been up for Halo 3 for a long time… But I can't aim somehow…

My question to you guys would be… How can I aim?

How am I supposed to hit the enemy without aiming… It takes way too long and the game is not really fun.

You can't aim normally with assault rifles. You can look in the settings but it should be on R3. This is the right stick pushed in (can also be L3)