Depends on what you want to draw and for what purpose.
Many architect/engineer firms use Google Setch Up (there is a free version) to do quick 3D renderings of buildings and sites, but, while it works to scale, it is not suitable for producing dimensioned drawings. Google has some tutorials and there is a "Sketch Up for Dummies" book available.
If you want actual production drawings with dimensions, etc, you will have to use a Computer Aided Design and Drafting program (CAD or CADD). There are some free packages, but I know nothing about those. AutoCAD and Micro Station are the big names for generalized programs, but they are expensive and have steep learning curves (especially Micro Station).
There are numerous clones available, and some are inexpensive but full featured (TurboCAD comes to mind). Do a Google search for CAD Programs. Plenty of program sites and program review sites.