I've found that a lot of users new to Android don't know how to switch to a custom keyboard once they have downloaded one from the market. This tutorial is a basic and quick way to show you how to switch to your new replacement keyboard such as Swiftkey or Swype.
1. First of all you want to go into your settings and find "Language & Keyboard".
2. You will then see your custom keyboard, mine is Swype as you can see. Hit your custom keyboard and make sure the box is ticked.
3. Then go into your messaging app and bring up the keyboard. Hold down on the box where you would enter your text message, when you have done this properly this box should come up with a number of settings. Go to "Input Method".
4. After selecting input method you should see "Android Keyboard" and whatever replacement keyboard you have installed. Hit on your replacement keyboard then you should be ready to go!