you never HAVE to bookmark your own posts... you can always find them by going to your profile and clicking on Find Latest Started Threads. that being said, you do not have a way to edit your own post after about 20-30 minutes. As Neesie said, you can only add additional information by using Reply on your thread, or just scroll to the bottom and use Post Quick Reply.
some members like to start a new thread and post a link in that to the old thread. in that case, you have to know how to post a link
. to do that, pull up the original thread [or any web page you want to refer to], highlight the entire web address at the top of the screen, right click and choose Copy. then, in your new thread, at the point that you want to insert the link, use right click and choose Paste. that's all there is to that.