Top thread is tight and bottom thread is too loose. How do you tighten the bottom thread?

You havent said what make and model this relates to.

But generally, you adjust the top thread and leave the bottom thread tension alone (unless it has been altered in error).

If your machine has a removeable bobbin case that goes into a front loading vertical rotary hook then you adjust a little screw on the bobbin case with a mini screwdriver. See this website for lots of great sewing machine tips and general help. http://sewing.about.com/od/sewingmachineparts/p/bobbincase.htm This page shows a removable bobbin case and explains which way the bobbin goes into it. If the bobbin is in the wrong way or not under tension this could be causing your stitching issues too.

Only 1/4 turn increments each time. To check tension on the bobbin case, put in the bobbin, pull the thread into tension spring, then hold the thread tail and dangle the bobbin case and bobbin (dangle test). It should hang in the air and the bobbin not release any thread. So turn screw up until you get it to this point but no further. With a gentle pull though, you want the thread to come out of it. Right to tighten, left to loosen usually.

If you search some of my other solutions I've covered the dangle test lots of times.