Hi Roland,
Littlehampton East Beach is goor for beginners. You also have Goring Gap, Lancing and Hove. You could try Joliffe Road in West Wittering but I found it a bit too rough there for a beginner, you could also join the Witterings Windsurf Club and use their facilities - great, sandy beach with a sandbank which produces some decent flat water.
I hope that helps.
Nick