The main thing is to make sure you choose the right subjects in school – Maths and Physics as a minimum. Also, work hard to get good grades. If you think you might be interested in an engineering field related to Chemistry/Biology bear that in mind and add those subjects. Material or design technology subjects are also often done by future engineers although I didn’t study those myself. Some business and/or environmental subjects would be useful because the best engineers are rounded people, understand the world around them and how the company they work in operates. Studying Business would also be useful if you decided to start your own company in the future.
As for things you can do now – in addition to keeping your grades up, if your school or local community offer afterschool clubs where you can do projects, participating in them will give you good experience. Ideally they will be engineering projects where you design/build/invent something but even if they are not, they will give you experience (and evidence for interviewers and assessors in the future) of working as part of a team, overcoming difficult situations, delivering a project to given timescales and within the limitations you are given (eg cost, physical size, etc). Even if there is no after school clubs already doing this type of activity perhaps you and some friends could identify a competition or challenge that you would be interested in working on together or do one on your own? There is a page on this website that highlights competitions and activities you could look at for a start, including the essay competition which you could do yourself.