How can you not know this? Learn how to create the most famous, and most common used photoshop-pattern there is... Scanlines. Awsome for signatures or photo-manipulations!

The most common used pattern all around the globe... learn it now!

• Step 1 Open up an image 1x3 and putt a black dott in the middle.(Background must be transparent)
  Click Edit>Define Pattern and open up a new image, no matter what size and background:
  
  
   
• Step 2 Click Edit>Fill and choose "pattern" from the menu and choose your pattern:
  
  
  
   
• Step 3 Nobody likes their scanlines fully black, so I'd suggest that you lower the opacity to 50% or 30%...

Tutorial Source