I started lifting around 14, and I regrettably started by using a machine. Barbells are good if you want to look cool and you want "OK" results, but dumbbells are far better for you. You have a greater range of movement to go through, and moving two independent weights challenges your coordination far greater than a barbell bench will.
Best thing I can say is the best workout is the one your body hasn't adapted to yet. Doing different lifts and mixing up your routine will force your muscles to work harder, and will encourage growth. Mix it up with incline dumbbell presses, where the bench is at an incline, to standard bench dumbbell presses, do some decline ones, and do alternating dumbbell presses, where you hold both over your head, and lower one and press that back up, and do the same thing for the other arm. Dumbbells are safer to work with than barbells, since you won't need to be so dependent on a spotter, and will help a lot more.
At 14, I would start with 25 pound dumbbells on bench, 15 on decline/incline, and work your way up from there. If it sounds small, just remember, everybody has to start somewhere.