Correcting your form is the best way to maximize performance and limit injuries in the gym. Lifting with poor form can lead to injury, including strains, sprains, tears, and breakages of muscles, ligaments, bones, and joints. Lifting lighter weight with proper form will always be better than lifting heavier weights with poor form. Proper form also means you will be working the muscles that you are targetting. Speaking from experience, a bench press with bad form can quickly become a shoulder workout.
There are many ways to become better at a given exercise, such as having a physical trainer or asking someone to teach you. However, many people don't have either of these options. Luckily there are HUNDREDS of very well made, short videos on the internet that can help you, such as this one from Genesis Health Clubs.