Q: I want to build mass in my thighs how can I do this and what should I eat

A: Building a lot of muscle in any area of the body requires two things: doing heavy exercises that target that area and eating a lot of healthy calories to allow for slow weight gain.

For exercises, while it's always a good idea to do Weight Training for your whole body you will want to prioritize exercises like squats, lunges, leg presses, and deadlifts (especially squats). If you are not familiar with any of them you can find them on my Essential Exercises page. Do as many as 40-50 heavy sets (meaning using weights heavy enough that you can only lift them no more than 12 times before needing a rest) of these exercises weekly, split into 2-3 sessions.

For nutrition, as I said, you need to overeat in order to slowly gain weight. Without weight gain muscle gain will be quite minimal. Make sure to read my Nutrition article and then put together a diet plan for yourself -- you can use Fitness Guide to get a starter plan. Then monitor your weight gain progress by weighing yourself every few days to make sure that you gain about 1 pound every 2 weeks on average (look for long-term trend since day-to-day weight will fluctuate but after 4-6 weeks you should see a clear trend). If you gain weight faster than that, then cut out some calories to prevent excessive fat gain and if you are not gaining fast enough, add more calories. Hope that helps and good luck!