A 3.8m board ?
Does that make it Equipe / Equipe II, or maybe a One Design.
I don't suppose you know roughly how old it is do you? Not that it matters, but things have changed MASSIVELY over the past 10/15 years. Boards have overall got a lot shorter and a lot wider. And I'd say learning's a lot easier now than it was when i first started
Anyway, bit of advice I would give is get yourself a nice warm wetsuit, a decent float coat/bouancy aid/whatever you want to call it. You will be spending a LOT of time in the water. You might even want to invest in a helmet if you can bear the thought of wearing one. I generally don't wear one unless it's seriously windy, but I can tell you from personal experience that getting a mast in the head ******* hurts

Oh decent wetsuit boots/slippers specifically designed for windsurfing = a good idea also.
Boards come with a nice non-slip surface... bit like sandpaper. Depending on the condition and age of your board yours might have all rubbed off, but you can buy a spray can of the stuff and make it as good as new - just need to let it dry properly of course.
That's your personal protection covered - I would see if you can get a protective cover for the base of your mast, and also the end of the boom that attaches to the mast (where the clamp is).. what am I talking about? It's/They're basically foam pads that clip/velcro onto the bits i mentioned, which in the event of you dropping the sail protect the board from hard knocks, which can (especially on nice boards like a mistral) put a dent/crack in the deck, which needs to be PROPERLY repaired. Best to avoid that if you can.
Did you get just one sail with it? What size is it (in terms of m^2/area) - will say on the sail/sail bag.
For learning you don't want something huge. I wouldn't really recommend anything bigger than 6.0m. I think I learned with a 5.0m (but I was a weedy little child).
I'll let you answer some of the excuse for questions I've put here, and then I should be able to offer more advice.
Available on msn if you prefer a chat.