This page explains how to make Ribs.

Because rib is a model based feature, you need to have some kind of geometry, in this example L-shaped extrusion is used.

Push Ribs from Features to start.

To use this feature, you need do a sketch on a plane, where you want your rib to be.

After you've drawn your sketch, you can move to the actual feature definitions.

You can define the width of your rib, the side where you want your rib to be, in this case "Both sides" is selected. This means that your rib will be "extruded" on both sides of your sketch at some width you define. You can select First side or Second side if needed. Extrusion direction also needs to be chosen. Your rib should look something like this now:

Then just finish your rib!