Ok, I am going to start Muay Thai but I am not very flexible. Could this really affect me because at this point I don't think I could perform a kick to the head.
I second that, kicks to the head aren't too common, as long as you can get as high as the ribs that would stand you in good stead. You can work on flexibility as you go. Have fun.
relax, flexibility will come with training. the more you do it, you'll notice yourself becoming more flexible as time goes on.
i was 300 lbs and could raise my leg only as high as to give knee level kicks when i first started years ago. after a year of training i had lost massive amounts of weight and could deliver head kicks if i wanted to. i ended up at about 200lbs, but my body composition has changed drastically. by looking at me most people underestimate my weigh because i carry a lot of muscle around my arms shoulders and chest, and my calves and thighs are mostly muscle, serious, they're like hard as rocks. it's only my mid/ core section that carries a sizely spare.
relax, just do it and in time it will come. trust me.
You can work on opening your hips and hamstrings though, because alot of people in muay thai have practical problems with the roundkick, impacting correctly with the shin. It usually has to do with the tight hamstrings or hip, the shin impacts flat on the target, especially for the bread and butter, mid line kick at the opponents rib, elbow target line. If you look at a photo or some video of someone doing the kick, you can see how flat the shin lands for maximum impact. In thailand, i know the trainers worked with many foreigners getting them to get their kicks impacting correctly. In time you work on the kicks and flexibility and you can go higher.
You can also use basic yoga dvds or classes, for hip openers, and various hamstring stretches you can work on over time to help with flexibility.
As has previously been said, head kicks aren't as important as leg kicks and midsection kicks.
That said, if you're looking to improve your flexibility, I was taught to spread my legs as wide as possible without doing the splits. Then hold in that position for a couple of minutes, do that every day and you will see results.
NB: I'm not sure how good for your legs this technique is, I was taught it by Panya Kraitus, he wrote the book, 'Muay Thai: The Most Distinguished Art of Fighting' so I gather he does know what he's talking about, but for all I know, it may be bad for your legs.