For graphic designs, pattern often time can be an important element. It is easy to use but yet can have a significant effect to the graphic. Pattern is a repeating element that can be used as a background or filling an area or object.

Creating a Custom Pattern in Photoshop
I will give a simple red polka dot pattern as an example.

To create a pattern you need to use Rectangular Marquee Tool (M) to make a selection. Keep in mind that your pattern will always be a rectangular regardless whatever shape you make. After making a selection, define your pattern by going to
Edit > Define Pattern

By doing this, you already have a new pattern which is ready to apply. To apply the pattern, select the area you want to fill, then click Edit > Fill, then select pattern you want to apply.
Just a tip, if you want to make a pattern, it is better to create it in a new file rather than making it in your drawing you are working on, so it will keep you work clean.

Creating a Diagonal Line Pattern
Make a new document with 5×5 pixels. In the Background Contents option, choose transparent, so it can be used in any backgrounds you want.

Use Pencil Tool and set it to 1 pixel from the option bar. Set the opacity to 100%. Because your canvas is very small, zoom it so you can see it clearly and bigger.
Make diagonal line from left top to right bottom. Here is how you do it : From the left top, click to create a dot, then move your mouse to the right bottom then while holding shift, click to make a straight line.

While still selecting your all 5×5 pixel document, define your pattern by clicking Edit > Define Pattern. (Note if your Define Pattern menu is not there, try to click Edit > Show All Menu Items.

Your Pattern is ready. Now it’s time to apply it to our design. I create a text with Futura XBlk BT font so just to make it bold.
Create a new layer on top of the text layer.
With a new layer still selected, Ctrl-click the “T” sign on the text layer. It makes the selection on the text.

Fill the selection with pattern with the pattern we just made.
Turn off the text layer, and add a white background so you can see how it looks.

Besides applying a pattern just it is, you can also experimenting with blend mode, opacity (from the layer pallet) and make a color change a little bit (after applying the pattern) to make different effect. You can also tweak the pattern by using Pattern overlay. With this, you can also have a lot of options including opacity, blend mode and also scale.

Applying pattern can bring different effect to a design. You can do more experiments to make different pattern and apply them to your text. If it is not enough, you can also download many free photoshop patterns available on the web. There are so many of them, both seamless or not.