If your hair really is coming out in clumps (i.e. you're starting to form isolated bald patches while the rest of your hair is OK) then it's probably alopecia. See a doctor! (It's a nightmare to treat but not exactly life-threatening!). [I won't recommend a web-site here. Type 'alopecia' into a search engine and you'll find thousands of them!].
For some reason, 'Immi666' sounds like a female screen name to me but, just in case it isn't, I'll mention that a large number (most?) men suffer from 'male pattern baldness' ( = the receding hairline thing). If this happens to be your problem, then you'll just have to live with it. (If you can come up with a solution to it, your fortune will be greater than Bill Gates' within a fortnight!).
If you're female, check out this link. (It won't solve your problem but it should convince you that hair los is a complex subject - so see a doctor!):
http://my.webmd.com/content/article/78/95827.htm
Lastly, consider just how many hairs you're losing per day. If it's around 100 then it's well within the range of 'normal' so you've got nothing to worry about anyway!
Chris