For those of you who are beginners at photoshop, this is a very simple task.
Download your font http://www.dafonts.com is a very good source for free fonts (or google free fonts) as some places you have to pay.
Download to your desktop.
Open up your WINDOWS file > Fonts folder > Drag your downloaded font into the Fonts Folder.
The fonts you have downloaded will look like either of these two icons, these are what you need to drag into your fonts windows file:
Photshop usually recognises the new fonts straight away but sometimes you may have to close|reopen the programme before you can use them, they will then appear in your list of fonts in the drop down menu, as shown below: