hi there, first post on the site.
take it from a fellow high-handicapper... take a lesson.
it is BY FAR the best investment you can make in your game.
you need a pro to show you the proper alignment, stance, grip, posture, ball position. these are not called the fundamentals for nothing.
if you don't have solid fundamentals, you will be buying plenty of training aids over the years to come, without seeing much improvement.
earlier this year i decided that i would enjoy my golf alot more if i was playing better. i decided to find a pga pro near me (i am from south africa), and have taken many lessons over the last 3/4 months. sure, it has cost me a fair bit (luckily my pro charges roughly $25, which sounds very cheap compared to the rates that has been mentioned in this thread).
now, if you can't afford more than one lesson, don't worry about it. go take that first lesson, and make sure to take meticulous notes of EVERY SINGLE WORD that comes out of the pro's mouth. this is vital, since you will only be having one lesson to start off with. once you have these notes, you can practise these over and over again.
these fundamentals are something you can practice at home. grip, stance, alignment, posture, ball position can all be ingrained without hitting any balls. now this seems counter intuitive to a hacker, i know i just wanted to start hitting some balls.
take alignment for instance, i almost always dropped a club at my feet when hitting balls at the range. but i couldn't see the alignment mistakes i were making, and because my alignment was off, i was making a subconscious over-the-top move that lead to my big slice. i would never have known the cause
but i promise you, you would be surprised how much EASIER it is to hit a golf ball once you have these fundamentals down pat. i can guarantee you that during your half hour lesson with the pro, you will already start hitting the ball better. you just have to make sure that you practise everything he says until it becomes second nature. this is the hard part, your grip, etc is probably ingrained already, and it takes a whole lot of conscious effort to not revert back to the old, incorrect grip, etc.
apart from the fundamentals, make sure that you practice your short game at least as much as your full shots (i am currently practising short game twice as much as my full shots, if not more).
to give you an idea, three months ago i was shooting around 110. i have already gained 10 strokes, and funny thing, my putting is worse now than when i began. i neglected my putting while working on the full swing, and am now paying the price for that poor decision.
have been working really hard on my putting, and i seriously feel that i can shoot very close to 95 in my following two rounds.
go take that lesson!!!
if you are still not convinced to take a lesson, then don't just waste your money on any random training aid. google for the y-drill and l-drill. these are short-swing drills, that you can practice at the range. no need for a training aid. but i fear without proper fundamentals, the results might not be as good as otherwise.
but, i still think the lesson would be your best investment!