Hardware and performance

We are currently students working on the mysql for my school's moodle website. According to my teacher we are to fix the runtime errors, however she is not very helpful. Can anyone give step by step instructions for fixing a slow_queries error starting from the very beginning? We are not very computer savvy, so the easier the better. If you know how to fix Handler_read_rnd, created_tmp_disk_tables, select_full_join, or sort_merge_passes it would also be greatly appreciated. We have googled and researched all of these topics but can not get our computer to give us the results they say we should be getting. Thanks in advance,

Nina and Larry