How to get the most from using weights ?

I have a membership to the YMCA and sometimes swim or do water aerobics. I also want to do some low-impact aerobics, though I have an ankle injury so I have to be careful.

I know the importance of using weights and would like to develop a routine I can do along with the cardio but I really can't afford a trainer to work with me.

Does anyone have ideas about how I can create a weight routine where I can start small and build as I go and it will be either free or low cost? Are there apps? Websites?

Thanks!