Owen, I could write you a book but I will try to be brief. Your grandfather could be key here. You need to learn what a good job is and what a bad job is. Before I ever built a house I spent about two years just visiting houses under construction after hours and looking at things. Most work is done correctly but eventually after enough looking, you will start to see the bad work. Your grandfather can show you these things. Work as a helper for a jack of all trades small handyman. You will learn a tremendous amount about how to do things and how long it takes. Remember at this stage your focus needs to be just not on learning how to do the work and doing it but building your knowledge base.
So much depends upon exactly what you want to do in construction. New construction? Remodeling? Commercial construction? Speciality trade? Initially your education should be tailored in one of these directions.
Being 18 is a double edged sword. The world is before you and your possibilities are limitless. Distractions and obligations are minimal and you can concentrate on your career. The downside is being taken seriously at a young age. People don't want to entrust a young person with their home even if he is capable and knowledgable. Old men like me have an advantage because our age justifiably or unjustifiably confers knowledge and capacity.You need to learn to manage men and clients. That is hard as a teen but you can learn some valuable lessons in other service businesses. Lawn mowing is a great example. Get you a truck and a mower and start mowing yards for people. Learn how to interact with homeowners and earn their trust. Expand and get another mower and hire guys to mow for you. Eventually you won't now yards but spend your time getting new business and managing the business of lawn mowing. You can do this now. People will entrust an 18 year old to mow their yard but not to reroof their house.
You can then begin to move into construction areas. Your mowers could become your winter construction crew when there are no yards to mow. Some lawn customers might let you do some home improvements. Soon you will have a customer base for construction projects. Sell your lawn business for a profit!