Take classes now in drawing and basic design. Learning to draw and having a good sense of design principles is essential. Learn to sew if you can. Knowing basic construction and finishing techniques is always a good idea.
You need to prepare yourself to gain admissions into either an art school with a fashion program like Parsons or Central St.Martins or a more technical school like FIT all of which require portfolios but of different kinds.
request admission catalogs or at least look up info from the following schools...
Fashion Institute of Technology
Parsons
Pratt
Art Institute of Chicago
Academy of Art San Francisco
Otis College of Art and Design
Central St.Martins
There are of course other fashion programs but these will give you an idea of what to expect as far as preparation and costs.
A lot of public state schools seem to have decent fashion programs. Kent State in Ohio and University Texas in seem to be alright.
If you want to study overseas in a country other than the U.K. learning a second language won't hurt. German if you are interested in the University of Applied Arts in Vienna, Dutch for Royal Academy of Art Antwerp, and Japanese for Bunka. French is good too as La Chambre de la Mode la Syndicale offers apprenticeship fashion and couture training that is done in french.