Have opened the program yet? After you download a program, you usually have to "install" it, this usually means double clicking what you installed. After that you open the program, and try opening a picture with it. Look at this page:
Image resizing, do it with PIXresizer. A picture 800x600 is a good size to upload to threads, not too big, not too small. Save as a Jpeg, if given a choice, most websites and email programs accept jpeg, and it takes up the least space. When you make a picture smaller, it's usually good to save it under a new name, or add the word "edited" to the name, so you won't ruin your original picture.
If you can't figure it out, there are many online sites that are easy to use, and will resize the picture for you. Here's one.
Resize Images online
Oh, I see V beat me to it!