You can type it out like I do, and that takes a bit of getting used to. Alternatively you could go here:
http://www.codecogs.com/latex/eqneditor.php and enter whatever you want then paste it back here and putting the tags [ tex ] and [ /tex ] at the beginning and end. Or a third method is type it out in the above link, save it as an image and upload onto an image sharing webiste and copy and paste the forum code they give you.