Hi, I’m new to this forum and to House. I have only seen two episodes. I was wondering, what’s wrong with his leg? I understand that it gives him pain, but why? Thanks

Here's the official answer from the FOX "House" website FAQ section:

Q: What happened to House’s leg?

A: He had an aneurism that clotted, leading to an infarction. Which basically means there was a blood clot in his thigh that blocked blood flow for three days before being correctly diagnosed. The delay in diagnosis lead to death of muscle tissue. His doctors recommended amputation, but House (being the ray of sunshine that he is) insisted that they simply remove the clot and restore blood flow. This option was dangerous because it would wash some pretty nasty stuff from the necrotic (ie dead) tissue back into his system—not to mention that it would be incredibly painful. The upside was that if it worked, he could regain full use of his leg. There were complications following the procedure and he was in extreme pain (like, maxed-out-on-pain-meds-and-still-screaming-like-a-girl pain), so he asked to be put into a chemically induced coma which would allow him to sleep through it. Once he was out, Stacy (his lover/health care proxy) okayed a middle ground procedure that would remove the dead muscle tissue, but allow him to keep the leg. We can debate the ethics of that decision until the cows come home, but for now let’s just stick with the facts—House woke up minus some thigh muscle, plus a limp and chronic pain.

Whatever you do, don't ask that question on the FOX boards!!!

If you have the chance, be sure to watch "Three Stories" from Season 1. It includes flashbacks to House during that period of time, and explains the whole thing.