Gary,
I second Richard's recommendation of Jem Hall's 'Beginner to Winner'. it used to be available online - I think there are still clips. If I were beginning now, I would purchase the retail version. Money well spent.
As far as progression timeframe; that's going to depend largely on how frequently you can make it out to the water, as well as whether you may be able to make any trips to places with conditions more conducive to learning.
Someone told me: "Once you start planing and can hook-in, try to make a trip to someplace with reliable steady winds". I think that was good advice. I'd even take it one step further. Make a trip as soon as you can to a place with steady reliable winds - regardless of where you are in your progression. Not only is it easier to learn with steady winds, but the same locations will also have the best quality lessons/ coaching.
When you're learning there ARE SO MANY things happening, and it's very difficult to sort out what's going on. Steady wind really simplifies things. Sure, you'll have to come back to the gusty conditions, but you'll have had a chance to get the feel for whatever goal you're working on (planing, foot-straps, whatever). Once you've gotten the feel for whatever skill your learning under ideal conditions, it'll make recreating your success in less-than ideal conditions much easier. Such locations are also likely to have good instruction.
That being said, the best way to learn is to get out as often as possible, ideal conditions or not. Don't let a marginal forecast keep you from trying to get out. Make sure you can proceed safely, but don't get stopped by not quite enough wind, or a little colder air temp than you'd like, or a bit of (no-lighting) rain. Don't sail in lightning, but also don't dismiss the entire day just because there's a chance of lighting or thunderstorms. Spend some time learning about weather and weather patterns. It's fine to just check out the wind forecast, but learning more about pressure systems, fronts and how to understand more comprehensive forecasts will help you get more good sessions in safely.
If you're confident you'll stick with the sport, then think ahead about gear acquisition. Even if you're not, make sure that whatever gear you buy is self-compatible (mast and sails). Even good shops have been known to sell a mast- sail combination that isn't really a good match (I've had this experience). There's no substitute for understanding this stuff for yourself - some folks might say it's intimidating or overkill to get into this sort of thing - I personally couldn't learn enough when I was getting started. Check out
www.ezzysails.com . Read all the info under 'masts' and 'rig support'. check out the videos too.
Before you buy any gear or any more gear, make sure you understand, MCS or IMCS (gross stiffness), AND different bend curves (t's best to deal with bend curves in terms of stats (1/4 and 3/4 flex points expressed as a percentage - basically these numbers just tell you how flexible the bottom of the mast is relative to the top). People will sometimes talk about the bend curves in terms of "constant curve" or "progressive flex" or "Stiff top". Those are three general categories, but they are imprecise, and as such, only make the issue more confusing. Insist on dealing with numbers, and things will stay much simpler. Don't rely on a salesperson to make sure the mast and sail you're buying is compatible - they don't always know, and until you're more experienced with how a sail should rig, you won't know either. If you're not up for learning about this stuff at this point; ask folks on the forum. If you go to a shop and they tell you 'it doesn't really matter, or "that's only important for OPTIMAL tuning, you'll still get 99% performance" Don't buy it. If they can't give you information on what type of mast the sail is designed for (length, IMSC or MSC, and bend curve (expressed as two percentages) or if they can't tell you the same info for the mast…. I would't buy it. It's okay if they have to look things up… but don't let anyone tell you these things don't matter. Windsurfing is challenging enough without having gear that can't be set-up properly.
I recommend the Ezzy website
www.ezzy.com/ as a concise source for understanding basic sail theory even if you don't buy Ezzy sails or masts (though you could do much worse, but maybe not better). I'd also seriously consider buying sails and masts of all one brand (or different sail brands that design around similar mast curves (I'd recommend Ezzy, but there are other great sails too). This will simplify things as far as making sure your masts are compatible with your sails.
www.ezzy.com/rig-support/ezzy-basic-sail-design-theory/
www.ezzy.com/masts/ezzy-mast-faqs/