How to become an engineer?

You need to concentrate on math and sciences in school. After you graduate, you would join an engineering program. What kind of engineering are you interested in?

There are several Engineering courses available nowadays.
All you need to do is score good marks in school. Especially in Science and Math's.
Then you can decide which stream you want to join.eg:- Mechanical, Electronics, Electrical, Computers, Information Science.... etc..
Once decided you can pursue your Graduation in that field.
You will be an Engineer once you successfully complete your graduation.

One of the old comments I heard: "Scientists explore what is; Engineers create what has not been". Generally creates an argument or two in the right groups! :)

I am a computer scientist by degree, and software engineer by experience. Engineering is all about seeing a problem, and filling a need. Sometimes you are part of a large team where the problem is already defined and you get your part to solve handed to you. Sometimes it is up to you to see the problem. :)

The comment above about focus on math and sciences is correct, since these are the areas where the solutions are often found. But two additional comments:
1. Make sure you ENJOY this. Not all the parts all the time... but you should have fun building and making things and solving problems. If you enjoy this, you will have a good career. If you don't... think about what you do enjoy and follow that. :)
2. Don't neglect your English, writing, and other skills. While the solution is often in the math and sciences, the problems you are solving are for people without your background! You need to be able to talk to them, write to them, and find out the details of the problem. It always sounds trivial, but It often times is not. You will also find that if you have an idea, you will need to share that idea with your co-workers... and a good idea expressed poorly will get ignored. Make sure to give your ideas a fair chance!

The world needs more engineers (which is why it is a good career). Enjoy exploring, and best of luck in what you decide! :)

Lots of ways to get into engineering. As others noted, math, science, the desire to understand the world and build things is pretty key. I have also heard that most engineers are optimists - wanting to always make things better in the world. I do agree. Consider programs like cooperative education or interning to get some real world experience. Get involved in professional organizations - student chapters of things like ISSA, IEEE will help get you exposed to the field of engineering and to professionals.