You should always have some songs you can play at anytime. When people find out you have a guitar, they will ask you to play something. You might not have access to notation you can follow, so get some songs memorized that you can pull out at any time.

When practicing riffs go slow and be deliberate. Make sure you are playing in time by using a metronome. Only speed up when you feel completely comfortable.

When flatpicking, make sure you pay attention to note duration and silence. Again, go slow and only speed up when completely comfortable.

Start playing these songs with strict fundamentals, using the proper fingers on the proper strings. As you progress, you can figure out the most comfortable way to play for you.

Rhythm guitar songs are all about strumming. But, this doesn't mean they have to be boring. Experiment with different patterns, or try your own patterns.