In game you press F8. There should be an image in the "Screenshots" folder. If not, something is wrong with your files.
And if you mean post a screenshot here, just click the "Go Advanced" button, go down until you see another button that says "Manage Attachments." In that window, find the file and upload it. You can do this with other files as well.

They say size is to big! Sorry for being an idiot. Its just I suck at computer stuff...

re size it with gimp or paint or with another editing program

You should try uploading the picture to an image hosting site instead of uploading it directly to the forums.
People will be able to see it full sized without having to click the thumbnail.

I would use Tinypic for upload.