First, for a router to be effective it requires a bit for cutting. Secure the bit in the router, by sliding the bit's shank into the collet in the router and tighten the collet nut. Don't push the bit in as far as it will go and tighten the nut, because most routers bits have a rounded portion at the top of the shank called a fillet. Tightening against the fillet will not allow a firm grip on the shank, and the bit could slip in use. Instead, insert the bit all the way, and pull it back out past the fillet before tightening the nut. Tighten the collet nut with either two wrenches or one wrench and a spindle lock, depending on your router model. Be careful not to over-tighten the collet.
With the router bit in place, adjust the cutting depth. Before actually routing on the workpiece, use a steel rule set on the router base held against the side of the bit to confirm the proper depth. And as always make a test cut before moving to your workpiece. Often a profile to be cut is too large to be produced with one pass. One of the biggest mistakes of beginning woodworkers is to try to cut a deep cut or profile in one pass. It is hard on the router and dangerous. Also the result will often be an ugly or burnt cut. It is easy to fix by using a series of passes at greater and greater depths until the proper profile is cut.
The right direction to rout is also a difficult concept at first. Moving the router in the wrong direction can cause it to grab the workpiece and even pull free from your hands—that's bad! Use the L solution to prevent routing in the wrong direction. Hold your right hand over the router in the backward L shape. Then point your thumb at the edge to be cut, your index finger shows the direction to move the router. Use the same technique to rout on the inside of a project. Again just point your thumb toward the edge to be routed.
Finally, make sure the workpiece is secured properly before routing. For light cuts a non-slip mat works well. For a more challenging cut use clamps or bench, but make sure they are not in the way of your router or bit. The router is the most used tool in my shop obvious by the number of different routers I own. Once you understand the basics of the tool, it will likely be the most used tool in your shop as well.