PJ pretty much covered the basics of submissions. Just remember that you are not just trying to win Tharg over with the quality of your work , but also with your professional attitude. While an Editor loves to publish great work they HATE getting stiffed by an amateur newbie who hasn't got their ducks in a row yet. So there is a great deal of caution when it comes to hiring untried talent.
As people have already suggested here, it's not one submission that gets you in the door, it's a campaign of submissions, fanzine art, convention meetings. Persistence and reliability will win you a great deal of favour. Also if you are willing to put this much effort into the chase, even if your work isn't up to scratch, Tharg and his minions will be aware of you and will pay attention as you improve.
A positive and professional attitude helps too. People like to hire people they like. That may seem obvious, but it's surprising how many guys come in with a chip on their shoulder or a sense of entitlement and it's very off-putting. Don't give them a reason to ignore you.
There are lots of good people here on this forum , so you shouldn't have trouble getting good advice.
Best of luck.
Simon.
