I am afraid of getting an infection after having surgery on my upper thigh with tissue expanders placed. What should I do?

No greater infx risk. I don't believe using tissue expanders during surgery confers any greater risk of infection than the surgery itself. They're used so that doctors can get a greater field of view at the incision site and have more room to work. Before the procedure, you're given a prophylactic dose of antibiotics to minimize infection risk.
Not too risky. Tissue expanders can be successful, with low rates of complication, including infection, deflation, and contraction from scar formation. Infection rates depend on whether the patient poses minimal risk for the procedure, whether there is proper planning and appropriate technique used. Expanders in the breast have shown less than 2% failure due to infection, which may or may not vary for the thigh.
It's natural. I think it's natural to have some fear, and certainly we all have a healthy respect for potential infection. Rest assured that your surgeon will be monitoring you for any early signs of infection.
Low-Risk. Tissue expanders are like balloons that are slowly inflated under the skin over weeks-to-months to effectively grow new skin that can be transferred elsewhere on the body. Since it is a foreign body left under the skin, there is a risk of infection. However, the benefits usually outweigh the risks.