These instructions are for opening a file with a name ending in .jpg. JPG is a graphics file format. When the file is opened, you should see a picture.
Double click on the file. If the jpg file is attached to an e-mail you should be able to click twice in quick succession on the file name or file icon inside the e-mail. If it's saved on the computer, you'll need to navigate to it's location, then double click on it.
If the jpg file didn't open right away, you will be presented with a window asking what program you what to open the jpg file with. To the right of open with there will probably be a browse button. Click on the browse button.
You can also right-click on the jpg file, and choose Open With>>>Choose Program....
Choose a program you want to use to open the file from the list.
All internet browsers will open jpg files. You are reading this now using an internet browser. Some common browser names are Firefox and Internet Explorer.