First you need to write a specification for the filter. What frequencies do you want to reject and what frequencies do you want to pass?
Armed with that information you then can proceed to design a filter. You will have to decide how much attenuation you want and how much loss you can tolerate. And, in the process, how many stages of filtering will be required.
There are lots of books on the subject. It's not something you can shotgun, as you have discovered. There are also cookbook calculators available online but these may not satisfy your needs.
Maybe a simple RC filter will do, or maybe you will need a five stage job with M derived end sections. Or a Sallen-Key active filter.