Disclaimer - Your question wasn't all that clear, so I'm guessing the shape you want...
There are so, so many ways to model the shape you're after, and which one is best will depend on your design intent. What dimensions are the critical ones? You want to be able to change those in one sketch and then have the rest of the model update if you need to change things.
I'm going to assume the rectangular cutout is the critical item, and that the wall thickness should follow this.
Hopefully the screen capture below is useful, and shows some tricks you can use. Let me know if you have any more specific questions - remember that drawings/images/etc showing what you're after will always be more helpful than words.