Best to start with the fundamentals course at learn.sketchup.com
The first lesson deals with that. Keep in mind that there are two way’s to create geometry, by ‘dragging’ (eg. holding the left mouse button) or clicking, than moving the other point in the direction you want and than click again to establish.
Last method is preferred, take good notion of how the dimensions are displayed in the measurements field on the bottom right, especially the ‘delimeter’ ( ‘,’ or ‘;’ )
If you’re not seeing the bottom status bar and dimensions field, maximize the SketchUp window at the top right (middle button)
If you want to introduce yourself, head for rhat topic and reply on the first post: