i don't want this to sound rude, but i have to say it.
you should completely read your manual for your carseat. When you don't know something as simple and basic like how to adjust the harness tightness, it makes me think you haven't read the manual. Which then makes me question whether or not the seat is installed correctly, being used correctly, etc.
If you have lost your manual, Graco will send you a new one, or they are available to read online at Graco's website.
Please read the manual completely, and become comfortable with your child's carseat...play with it a bit....misuse can be just as dangerous as not using a seat at all.