As mostly stated above, it takes practice. The more practice you get by creating new things, the better you will become. It's just like any other trade almost, it takes time, dedication & imagination to be a designer.

Take some PSD files for different industries or type of websites and edit them in photoshop and see what you can create. Then try it from scratch once you feel comfortable enough and have some imagination of what you wish to design. Believe me, once you get the hang of it - it will be so easy.