Hi Liz
I did this trip this time last year - You'll have a blast.
If you type 'Iceland' into the search function on the top right hand of the page, you'll find all the threads containing reviews, hints and tips, including those from myself and the JY Iceland Class of March 2015 including SallyC, MikeW and Bob Gray.
My main tips would be to take Yaktrax or similar to pop on to your boots because it can get really slippy underfoot, especially on the Golden Circle trip (an absolute must!). Don't bother taking a brolly because you'll never keep it up and you'll just get distainful looks from the locals. But do take some thick gloves and a scarf or balaclava as the wind can be really biting, especially if you are out in the middle of the night Northern Lights spotting.
Finally, keep all of your till receipts because you will be able to reclaim the tax you have paid at the airport. Shops will give you an envelope to put them in which has the instructions on it about making a claim and the counter at the airport is well sign posted.
Happy to help if you have any other questions
Have an amazing trip
Mandy