I am 15 years old and want to become a web developer/designer. What are some of the first things I should do to start?

Start developing and designing web content on your own right now. Teach yourself.

There is a difference between "web developers" and "web designers".

Download the free tools and start practising. Web "developers" make good money if they can do Linux work under Apache.

Web "Designers" make almost nothing. Beware of any job with the title "designer" in it. The word is used to make the job sound "sexy". "Title escalation" is common these days. Schools use it to lure in students and command very high tuition. Do not spend four years, and 100K to get a job that pays only 40k.

Besides web developers are nut cases. Why do you people create forms using circular logic, forms that fail to validate if some read only field is left blank, or the other crap you people do? Honestly, if I have to view the HTML just to figure out what your form needs in order to validate, then you have done something wrong.

Check out free learning sides like the one below. They're a fine way to get started. With that as a foundation, go to the library and pick out books on the developing subjects that interest you.

Then get good grades and get a full-blown degree. Self taught developers don't get as far any more.

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How about reading?? I'm sure the book stores and/or library has books on this.
Web Site development, Java for dummies, Web Design All-in-One For Dummies, etc.