ok this may sound stupid but how do I mark a best answer??

₪ For Your Questions:

You must wait 4 hours before you can pick a best answer. Go to "My Profile" where you will find a list of all your questions. Select the question and there will be a "vote as best answer" button. You will have 4 days to pick one or it will go to a vote. You can also force the vote to start immediately on the edit (pencil) menu.

₪ For Someone Else's Questions (and your own after 4 days):

You will have to wait 4 days if the asker did not pick it himself. There will be a "vote as best answer" button. You must vote before the vote closes after 2 more days. You are allowed to vote for your own answer. You just can't answer your own question. Look for questions you can vote on by clicking on "Answer" at the top of the screen and then "Vote" on the tab.

₪ ɦəlʞɹɐq ₪

I can't explain this any better than Barkley Hound. So, come back and select his as the Best Answer to this question.