What would I tell my 15 year old self?
Yes there are options where everything is given to you on a plate, but they are few and far between so if you want to be a pilot, plan on paying for it yourself and you won't be disappointed.
Step 1: Get a medical!!
There is no minimum age for getting your aviation medical. That is the first thing you should do. The initial is a very big deal, they check everything and it costs about £600. You MUST get a class one if you intend to fly commercially. People will probably try to tell you you don't need one to get a PPL etc - which is true - but there's a huge difference between a class one and a class two - basically you need to know sooner rather than later IF you can pass one. The good news is that the renewals are much cheaper and easier. You can let a class one lapse by up to 5 years without needing to go through the initial procedure again.
So start saving, get a weekend job, put it on your Xmas/birthday list etc...
Step 2: Make a plan.
Once you've got a medical you can start planning your future. Do you need a degree to be a pilot? Definitely not, although as others rightly say it's usually better to have one than not. Unless you're in 1970s Cambodia.
If you get a Degree it's going to take 3 years and cost £60,000. A frozen ATPL can be had for £40,000.
If you spend those 3 years working, saving and flying part time then the cost of the fATPL will drop considerably. If you spend 3 years doing something where they clothe you, house you, feed and pay you (like the Military) then you'd be fully qualified by the time you left. A pilots licence is mandatory for being a pilot, a degree is not. You can always get the degree later.
Of course, if you really want to make the big bucks you could become an aviation engineer!