if the cable is frayed, change it out or bring it to a bike shop where they will do and set it back up. also make sure the teeth on the cogs are straight and not bent. it may be an issue. if i was you i would start changing out cables, and lubing things up. there could be multiple issues but we've got to start somewhere, so start with new cables if the old ones are damaged. one more thing you may have set the tension too high on the rear derailler cable and it needs some more slack, keep playing with it and watching youtube videos, you'll get it. and start learning names of things, we can help much more when we know exactly what youre talking about.