if i want post some image i have in my computer, how to do it?

You need to have your pictures/screenshots available on a web site in order to link to them from your post in the forum. There are many free picture hosting sites, such as www.imageshack.us or www.photobucket.com that you can upload your pictures to. Once you have uploaded your pictures, use the hosting site's image link to your picture and paste it into your post. Most hosting sites have tutorials that explain how to link to your pictures.