Manual focusing on lens?

I'm getting nikon d3100 with kit lens, and the 50mm f/1.8… & I'm wondering, how hard is it to learn how to manually focus on the 50mm? I'm going to just learn myself, but just wondering what i should look forward too, since I'm a beginner

Quite tricky. The focus screens of the modern cameras do'nt make it any easier.

There's an electronic rangefinder in the viewfinder. You can manually focus with ease, once you learn to use it. After all, photographers have been manually focusing lenses for decades

The DSLR focusing screens are not as good as many of the old mechanical film camera screens for manually focusing. Those had split ring and microprism focusing aids. The electronic indicator scale of the 3100 should get you pretty close to what could be done with autofocus. It is using the same detector that the autofocusing system uses, only the indicator is in not quite as fine an increment. The 50mm is very sharp to the edge if you stop down to f4.5 or smaller, and that will have enough depth of field to be forgiving of small focusing error. If you are trying to get the dramatic f1.8 only-the-eyeball-is-in-focus effect, you might miss the best focus with manual focus more than what autofocus could do.
And if you're doing self portraits you're SOL with the manual focus.