Many online forms, especially job applications, have a place for you to upload a file, usually a resume or other supporting documentation.
There will be a button that says something like Choose File. The paperclip is another indiciation that the form will require an attachment.
Also notice the types of files that the form will accept. Make sure your document is saved on your computer as one of those types of files.
Click on Choose File. A new window will open and you will be prompted to find your file on your computer.
Navigate to the file on your computer, click it once to select it (make sure it is highlighted in blue and listed next to File Name at the bottom of the window), and then click Open.
When you see the name of your file next to the button, you will know you have been successful and you can continue completing the form.