You will get a bit of rain, a bit of snow, varying degrees of wind and some sunshine (if you're lucky). That should cover your first hour in Iceland. Your next hours and days will be the same, but probably in a different order and the temperature will fluctuate accordingly. The weather will change without any apparent warning, and you've got to be quick off the mark with the layering-up or layering-down!
Basically, it'll be very changeable but the weather will come mainly from the cold side of the spectrum.
Wear layers - bring waterproof over-trousers if you can, and ice grips are useful for the Golden Circle trip. Whilst it will probably rain at some point, don't bother with an umbrella unless you want to do a Mary Poppins.
I did this trip last March and it was one of the most enjoyable holidays I've ever had - there is a very good review thread contributed to by various people from that trip and you'll see that we all enjoyed it despite (or because of?) the weather.
Enjoy your trip, and be prepared for anything weather-wise (except a heatwave).
