I got up means I got out of bed - usually to start the day. I was up means I had got up earlier - I was already out of bed. I got up at 8. - I got out of bed at 8. I was up at 8. - I had got up some time before 8. There's also 'woke up', which means that you stopped sleeping but you didn't necessarily get out of bed. I woke up at 8 and read in bed for an hour.