Make sure you dont have the resistance right up... Around 3-4 is fine.
Try the
Pete's Plan which has a 24 week programme to get going. As it says, if you can't do 5km from week 1, just follow the plan but half the distances or time e.g.
Week 1
Day 1 = 5000m
Day 2 = 6 x 500m / 2min rest
Day 3 = 5000m
Optional Day 4 = [20min]
Optional Day 5 = [2 x 10min / 2min rest]
Becomes
Week 1
Day 1 = 2500m
Day 2 = 3 x 500m / 2min rest
Day 3 = 2500m
Optional Day 4 = [10min]
Optional Day 5 = [2 x 5min / 2min rest]
Once you've built upto the 10k week (which becomes your 5k) you can start at Week 1 again and do the full distances/times