A plethora of tutorials exists online and it's easy to get sidetracked. I recommend tutorials by Aidan Chopra, Trimble's charismatic blogger and author of SU for Dummies. Any tutorials from SketchUp are helpful, esp if you take them in related groups, in small doses. There's a cluster of SU Basics called the toolbar series that takes you thru the commands, tool by tool. Pictured is the large tool set, the fat crayon of SketchUp.
1. Make the GUI style to your liking! At the boot-up window, I recommend the engineer's template over the others, or one that has the ground plane lightly toned so the horizon always shows. You can change SU's default color backgrounds and fat profile lines.
2. SU has a dialog box called Instructor, activated under the window menu. Several of my students got up and running with that alone.
3. Keystroke commands are worth printing out and learning from the git-go. Tool modifier keys are suggested by Instructor.
4. Once you start modeling, the first thing is to figure out how to keep things from sticking to each other by using groups. command G
5. Keep it simple. Make simple shapes, group often, alter and arrange groups smoothly, size them; then create and save views. Beginners: avoid the black hole/time sink of snazzy Google Earth functionality, picture matching, or pouring on SU's factory-made texture/colors, and you'll learn fast. Components can come later. Better to make good shapes and learn how to organize and manipulate them deftly before having them jump hoops or wear gowns.
6. The instant you learn groups, you have the opportunity to acquire a great habit: name each group (entity info box) and watch where SU puts it in the Outliner box. Every extra second you invest in doing that will be repaid sevenfold later on, so start the habit NOW. The Outliner box is key to efficient modeling. SU has LAYERS*. Do all construction in Layer 0. See tutorials on this. Outliner is sitting there just waiting to show off its stuff.
7. Learn to use the inferencing function (no key to press, it's always there -- at times a little too eager to help)
8. Sketchup is vector-based; just points, lines and planes. Look for the green end points. They count for more than they appear to.
9. Soon after you fall in love with SketchUp, consider buying the pro version. Its cost is a fraction of the competition's. The Pro version shakes hands with far more colleagues & includes nifty document-production functionality in Layout.
Everything I mentioned appears in the pic above except for the boot-up window and the green end points.
*layers' functionality is different, and useful, but less so for the beginner.