How can I make sure my baby does not become overweight?

Be feeding smart. If babies are otherwise well, they are pretty good self-regulators and will let you know when they're hungry and when they're full. If your baby is growing well, don't push. Many pediatricians also recommend no juice at all or at least limiting it to 4-6 ounces per day. The limited nutritional calories and the encouragement of a "sweet tooth" aren't good and can help set a path toward obesity.
Nurse. There's no way to guarantee a healthy-weight baby, but some good ways to help include nursing, paying attention to the baby's hunger and satiety cues, introducing solids at 4-6 months, and eating meals together as a family as often as possible.