You need to be in sketch edit mode and need to use relations/dimensions to align them as needed.
In the first rectangle sketch, add a diagonal construction line from corner to corner. In the second do the same. Right click the line and 'Select Midpoint' hold Shift and select the diagonal line in the first sketch, add a midpoint relation. Now the second is fixed to the centre of the first.
That'd be my preferred method, but I don'r know where they're at in the model and didn't want to suggest deleting and adding new sketch entities.
While in the sketch, you can use...
- Linear Sketch pattern
- Circular Sketch pattern
- Draw and place construction lines and use Symmetric relations
- Make your shapes blocks
- Draw/dimension/relations each rectangle separately Align one with the origin (dimensions or relations) then align subsequent shapes using dimensions and/or relations.
As for the plan, you should be able to achieve that form in 4 features. Extrude, Extrude Cut through, extrude cut though, extrude cut blind.
PS, have you done any of the tutorials yet?