I couldn't agree more with Gwen.
Just Do It! Take up Horsemanship for one thing. (Please don't be one of the "Joust Jocks"* who just want to get on a gigantic horse, hit the spurs and go, then get off [one way or another!] and repeat as necessary.) As Jeff is showing folks in his "Knight School", you need some basics first. Walk before you Run.
I would recommend first English riding lessons, for the seat, ballance and use of the legs. Learn to ride without stirrups at all gaits. Then after you are reasonably competent at that, go for Western lessons, as the neck reining, longer stirrups and different seat are both useful and necessary to become a complete Horseman for this game, or Early-Modern Cavalry for that matter.
Ground School too, learning the Lance from the Dirt up. (Beats learning it from the Lance down to the Dirt!

) Again, take Jeff's lead on this: learn from them what's been there, done that if you possibly can.
While you're working on that end of things, you can work on upgrading the armour and soft kit!
Sure, it takes some money and lots of time, as you aren't going to be doing any of this in a month. Or a year, even. But a long-term focus on your goal, with a budgeted amount of time and money will get you there in the long haul. If you have a five-year plan (remember, Squires had a longer apprenticeship than that) and stick with it, you ought to come out the other end quite the accomplished (and well accoutred!) Man-at-Arms, with LOTS of Moral Superiority to lord over your friends!
Good luck in all of this, and God Speed!
Allons!
Gordon
* BTW, there are PLENTY of excellent jousters out there who are anything but "Joust Jocks", who spend countless hours on their horses working with them to improve both their own horsemanship and their horse's skills and training. But sadly, there are a few who do not, and they can be rather a blot on the whole at times. My snide comments are directed at these few.