Here is a tutorial I made for beginners on how to make non-animated stamps with Photoshop Elements or Photoshop. The company Adobe makes Photoshop Elements and Photoshop. Elements is a simpler version with a lot less features.
Feel free to try different techniques to make stamps. There are other tutorials on deviantART, some for beginners and some for more advanced users. Some tutorials have methods that are very different that the ones I use. Here are some other tutorials other people made on deviantART:
Making stamps with Paint or Paintbucket [link]
Making stamps with Photoshop 7.0 [link]
Making animated stamps with Photoshop 7.0 [link]
Making animated stamps with Photoshop and ImageReady [link]
You can choose your own stamp template or make your own. There is a stamp template at [link]