After you find a website with something cool you want to pin you need to go to the Pinterest website logged into your account. Click the button "Add" in the upper right corner of the screen.
Then click "Add a Pin"
A place will pop up where you put in the address of the item you want to post, and then click, "Find Images"
Then click the two buttons "Prev" or "Next"until you get the picture you want. Then select the board you would like it to go on. You can type in the description you want, and lastly click, "Pin It"
There you have it. The tutorial on how to pin things from the web.
Hope this was helpful! Comment if you still have questions.