The number one thing is to get your water tested for hardness. Everything depends on this number. Pay for an independent water test. It cost me something like $55 to have a complete water analysis including all electrolytes, total dissolved solids, grains of hardness, and coliform count. Just a hardness test would be cheaper.
Once you have this number, now you can figure out the size of softener needed. We have hard well water, 38 grains, so we needed a 70,000 grain softener for 4 people. It regenerates at 1200 gal of water use, and I have it set to do so at 2:00am. It's got a plug in meter controller. Local dealer builds them himself here so it's not a name brand, but he uses excellent quality parts.
The other thing to keep in mind is not to size it too small. Salt use versus hardness is not a straight line relationship (geometric not arithmetic progression for whoever remembers high school math

) So if you install a too small softener, you will actually use more salt than a larger one. My 70,000 grain softener only goes through one 20kg bag of salt in about 6 weeks, 2 adults, 2 young kids, though we do have water saving appliances.