Qualifications
Welding requires highly specialized skills, so you can't just walk onto a job and expect them to let you start torching things.
You'll have to go through some form of training program. If your high school partners with a vocational school, you may be able to receive basic training while still in high school. Otherwise, you'll need to go to vocational school or community college and take courses in welding.
Many companies offer internship and apprenticeship programs, which offer the opportunity to build the technical skills necessary to become a professional welder.
Alternately, the U.S. Army offers a welding training program. But you'll also need to be in the army, so keep that in mind.
But regardless of where you receive your training, all welding requires a few fundamental non-technical skills. First, you have to work deftly with your hands. If your hands jitter like a caffeine-addict rodeo cowboy riding a jackhammer, then welding is probably not the right career for you.
You also need to be physically fit. You don't necessarily need to be a UFC fighter, but a big part of the job is lifting big ol' chunks of metal and holding them steady. And you also need some mental fitness. Let's face it—the job can get repetitive. Six hours in to fusing rectangles together, and your mind might start to wander. Don't let it do that.
You aren't necessarily required to be certified to work as a welder, but certification makes you more employable. And many higher paying jobs will require the certification.
And if you're considering an exciting and potentially lethal career in underwater welding, well, you're gonna need a whole other crop of qualifications. For one, you'll (obviously) need to be certified to dive. You'll also need to be able to withstand freezing temperatures, extreme conditions, and shark attacks.