Training your junk e-mail filter can help you save time and energy when dealing with spam. Outlook's built-in spam-filter is pretty good on its own, but it's not 100% accurate. There are several things you can do to help train the filter.
Open up your junk e-mail folder. Look through the folder. If you find a message that isn't supposed to be there, right click on it and hover over "Junk e-mail". It will look like this:

There are several options here for training the filter.
Add Sender to Blocked Senders List - This will make sure that any e-mail from this specific sender is always marked as junk e-mail. This isn't all that useful because most spammers never use the same email address twice.
Add Sender to Safe Senders List - This will make sure that any e-mail sent from this sender is never marked as junk e-mail.
Add Sender's Domain (@example.com) to Safe Senders List - This will make sure that any e-mail sent from this mail domain (@iastate.edu for example) is never marked as junk e-mail. This isn't usefull or recommended because spammers can put any address they would like on the e-mail.
Add Recipient to Safe Recipient List - This will make sure that any e-mail sent to the specific recipient is never marked as junk e-mail. This might be useful if you are subscribed to an e-mail list and that e-mail list is listed as the recipient of the message.
Mark as Not Junk... - This will bring up a new window that will look like this:

In this window you can choose to always trust messages sent from this sender and you can choose to always trust messages sent to the recipient. As a rule of thumb, it's ok to trust the sender, but not the recipient.
Clicking OK will move the message to whichever folder your message sorting filters specify.