Assuming the action is right on your guitar and you aren't using too strong a string gauge.
- Press down on a note like you normally would and play it.
- Lighten that pressure and play it again
- Repeat that until you can't get a clear note ringing out. That's the quickest way to get your minimum pressure.
- You can repeat this process with your standard open chords.
- To help with that, I'd suggest just strumming an open a minor and then switching to a c major (yes it's just a single finger change - fret 3 on the A string)
- Once you're comfortable with that you could try something like an e minor to g major. That'll get you used to switching your hand positions (and in turn make you go lighter with the pressure because you'll find your speed increases the lighter you go).
With respect to being able to use all your fingers. That comes with practice. Working through the standard pentatonic scale will help you start using all four fretting fingers. You could also consider doing the three notes per string style major scale, that's good for getting you used to using your pinky and 2nd last finger. There's also things like the spider warm up exercise and variants that help you use all four fingers (or get used to them).
Good luck !