I am very worried about my knees this prbleme started as no big deal but has now started to worry me. If I stand in one place for any more than two minutes they feel as if they are going to give way! And if I sit with them bent it also hurts. I am a male aged 18 I am not over weight or big built I don't do any sports or heavy lifting so there is no obviouse cause for this probleme as far as I can see but hopefully some one can help me or give me some advice.

You need to see a doctor because there could be many different causes. Water in knee, arthritis or muscle strains. Or etc...

We could all sit here and guess but the only way to know for sure is by actually getting assessed by a doctor.