Get your provisional sorted nowish. I waited around a few months and seriously regretted it.
Theory test is easier than I thought it was going to be. Whilst it doesn't hurt to learn braking distances/Random Traffic signs, etc, REALISTICALLY you should be able to scrape through the test with common sense alone. Please note in no way that I condone you not revise the highway code, I'm just saying that you might not need to devote days upon days memorising it from cover to cover to pass like some people believe is necessary.
Driving instructor - Get some recommendations and get a good one. I seriously lucked out with my instructor who genuinely wanted to see his students succeed. Put it this way, my first lesson was with a random guy through a small franchise that was doing a free half price lesson, I did the lesson, came back home (learning nothing) and he said that I would need at least 40 hours before I was ready to book my test, and that if I bought 25 hours now he'd give me a 10% discount on the 15 others. Okay then... I didn't like his attitude when I saw his face drop when I told him I was booked in for a Half price lesson. I then went on a lesson with the guy who would become my instructor. After my first lesson with him (second in total) the guy told me to book my practical as soon as I got indoors because I only needed a few hours of touching up that he could find inside the few weeks waiting time. I passed my test with Zero Minors 15 instructed hours later

. He could have told me I needed the 40 hours and dragged out the process, but didn't, and I'm grateful for that. This is what a good instructor should do
I have heard rumours that the theory test/practical test structure may be changing soon. It may be rubbish (probably is), but I'd still get your test sorted out as soon as possible just in case.
Insurance - It'll be very, very, very painful. Even more painful if you don't do your homework, so I'd start getting quotes TBH.
Tax - Once a year (if you choose). Worth noting that cars with a displacement with 1,549 (1.5L) will quality for a lower tax band (at least on pre 2001 cars), but chances are your first car is going to have a sub 1.5L engine anyway!
Mot - Once a year, but make sure that you get a car with as much MOT as possible. While it's not a iron clad guarantee that a car will be in good condition, it certainly helps.