When your fingers get sore, lay off until they are healed, then start again. They will harden over time.
As for strength in your little finger, again it is all down to allowing time for your muscles to get used to what is being asked of them - practice and exercise.
There is a 'but' to the last bit though - it is a common mistake when you start to play to apply too much pressure to the strings to fret them. Guitars don't need heavy pressure - basses a little more, but it is the fret that does the job, and you don't have to press very hard to get the frets to work.