Can't get a grip on the S

Be very careful about over stretching on the lower (1-5) frets. This can cause injury problems. Use your thumb as a pivot. The video below shows good left hand technique, including how to pivot. Also, check out the "How to get started" sticky for lots of general tips/advice.

How to get started?

 

Practice, lots of practice. To begin with, start out up higher on the neck where you don't have to stretch as much. For example, place your index finger on the 10th fret of the G string. Concentrate on holding your index finger on the fret and then place your ring finger on the 12th fret while keeping the index finger in place. Work on that until you feel comfortable. When it feels good add the D string on the same frets. Do this until you can do that on all four strings. Then slowly drop down two frets and start all over again. Then down two more frets, etc, etc, etc. This should gradually increase you reach and imrove your technique.

 

Good advice above. Two other thoughts: First, it's perfectly fine to shift you hand a little to get to that second note: When you fret the higher note with your third finger it no longer matters where your first finger is. Second, a common technique when playing down near the nut, where the frets are farther apart, is to use your third and fourth finger together as a unit, so (for example) you might play F with your first finger, F# with your second finger, and G with your 3rd/4th fingers together.

 

Yes. In the lower positions, using the third (ring) finger at all is unnecessary. It is the least independent finger. You may use your pinkie instead.