Keep plugging away. It is deceptively simple. There are tons of tutorials on the web.
For example, with the rectangle tool, draw a rough square and then immediately type the dimensions you intended which appear in the bottom right corner of the screen. Don't bother trying to interact with the dimension box, just start typing as soon as you've placed the second point. If you made a mistake, just retype the dimensions. It will take fractions as well as metric units.
Dimensions are accomplished by selecting Dimensions from the Tool menu. Click on the end points of what you want to dimension.