If you are just going along with the thread, just hit "Replay" and type happily and if you wish to attach a picture go down a little further and you will find a tab, " Manage Attachments", click on it, and were it says "Browse" click again. Your file tree will appear and there you look for the file you want to attach. After selecting it, you click "Upload" wait a few seconds and it is done. If there are more pictures you do it again up to three, and those no more than 97Kb each. If you have to downsize go to a recent thread, "Downsizing images" and you will find answers there. What I do is to open a file on the Desktop of my PC, I call it Forum, in there I open two files, one for text, the other for pictures. Once I am satisfied with both and ready to post, I just Copy and Paste the text and upload the picture.
If you want to answer to someone particular person, instead of clicking "Reply" you click "Quote" and follow the same routine. If in the top menu bar you hit "User CP" you will find how to insert your signature, avatar, edit your profile, etc...