When it comes to strength training, whether you are lifting dumbbells or using machines, correct form is really important. It doesn't really matter if you can do bicep curls with 30-lb. weights if you're struggling and grunting and can't keep your back straight. Keeping proper form will not only prevent injuries, but it will help you to isolate the muscles you're trying to work in each exercise.
You want to challenge yourself, but you shouldn't be straining or in pain. To do this, start off with a lighter weight and be sure you can do all 12 to 15 reps correctly before gradually increasing to a heavier weight. As your muscles get stronger, you'll be able to lift heavier weights with proper form.