In ModelSpace, start the Viewports command, toggle the Setup to 3D (lower center), now to start try the Three:Left choice in the left hand box, pick OK. It is easier to work in 3D if you have several views to follow so you aren't working away from the model.
Next you will need to become very familiar with manipulating the UCS. So start with the UCS command and look at the options at the command line. The option OB will let you select an object to move and align the UCS to. The option X, or Y or Z will let you specify an angle to rotate the UCS so you can align that to a plane you might want to work on.
Notice the DUCS button at the bottom of the screen, this is DYNAMIC UCS. You may want that on at least most of the time as it will align the UCS for you as you work. Sometimes it gets in the way, so just toggle that off when needed.
Also, look to the upper left of each viewport, you will see choices for views and for visual styles, pick on one or the other and choices pop down letting you change those easily right there.
Turn the Ribbon on if it isn't already and start the 3D Modeling Ribbon, start learning the commands located there.
PressPull (and Extrude) will be the two most basic commands you will be using for solid modeling.