Most members thank the giver for the giftie within the thread where the freebie or gift is announced. When you're viewing the giver's post, you can scroll down to the bottom of the page, and click on the left hand pink button "Post Reply" just above the green box for the quick reply. That gives you a window in which to put your comments, add smilies, etc. When you're done writing, click on the green button "Submit Reply" below the box showing smilies & icons. That's all there is to it!
If you want to know how to post a PM reply thanking someone, that's another lesson
. Let me know if that is something you want to know, I'll be happy to help out!