SCRUM how do you estimate time - in hours??

Hi, I need to give time estimations for my tasks in hours! I find that ridiculus - its SCRUM, how can any task be made in hours? every task requires defining requirements, analysis, design, documentation, testing, automatic testing. Should I put a note on each of these tasks? and estimate it with hours?! unittest1() - 1 hour? section A in document 1 2 hours? what if someone calls me and asks for help? what will happen to those 2 hours?! also can someone give me more tips how do you estimate your time?! I find I always underestimate my time (as well as anyone else I know around me!) plz help...