It depends. If I'm just taking photos outside or something, and there's plenty of light, I use Av on my Canon - "A" on Nikons - which is for Aperture Priority. This lets me use a lens wide open, if I want a shallow depth of field, and the camera chooses the shutter speed for the right exposure. Still need to set ISO (you also do this in P mode 'program mode') and if outside and it's bright, I'll go with a low number like 100 or 200. If indoors and with less light I'll use 400 or 800...
Really - if you don't have a really good understanding of how the 'exposure triangle' works - you might want to do your important photos on the automatic modes or the full auto setting. This way you can learn at your pace and let the camera do the thinking for your important photos.
Good luck!