If you are commuting, I advise you to get fenders and a rack with panniers. Using panniers takes the weight off your shoulders making for a more pleasant ride. Do NOT JUMP curbs, this is not a BMX bike. As others have said, bike fit is everything. I would not worry about going fast for now. Commuting is about doing it regularly day after day. Its far more important to be safe than going as fast as possible. Therefore, invest some money in multiple lights and a visibility vest (dollar stores have them for cheap). You did say its for commuting right?

After riding it for a year, you will have a much better idea of things you want to change. If your wrists hurt after an 18 mile ride, you are probably leaning too much forward, consider a shorter stem, or an adjustable stem or a handlebar that sweeps backward a bit more. If you want to go fast right away, changing to lighter bald tires is the way to go although given the season of the year right now, that might not be a smart thing. At 14, I understand that speed is important to you. But as a commuter, you need to think safety and reliability of equipment first. Hope this helps.