The first advice I will give you is that you need to get a good kit. Test strips aren't gonna work for you and although useful at times, an OTO kit isn't going to give you enough information to clean up your pool. The kit we recommend is the Taylor K2006 o 2006C (better buy) that you can get through the link in my signature below. Do yourself a favor, and get that kit. It will make your life much easier.

As far as sand vs. cartridge, you'll hear pluses and minuses on both depending on who you ask. But, either way, you have to kill the algae and in order for us to advise you of the best way to do that, we need some good testing numbers.
Welcome to the Pool Forum!