It's great that you're interested in commercial aviation! I was hooked when I was able to "job shadow" my dad, who is now a retired airline captain. While some things have changed, as far as security is concerned, after 9/11, there are still ways we can show you the career and get you out to the airport and to a plane. Let me know what major airport you are close by and I will see what I can do. There are also organizations such as Young Eagles, AOPA, and the Civil Air Patrol that you can join. Either way, I'd be glad to help in any way that I can. Good luck and blue skies!
The best route is finding somebody that you know that can connect you with a pilot. Because of regulatory limitations, commercial airlines may be a bit harder to get an opportunity to shadow for.
An alternate step would be to find a local aviation club at a small airstrip and call the front office there. Asking about shadowing opportunities is usually supported, welcome, and easier to find at that scale.
The best way I know of to shadow a commercial pilot is to get involved in an internship with an airline. By doing so you'll have a company ID which will be required to access the aircraft while being escorted, and many airlines who run internship programs will also have mentors who can show you what each job at the airline entails, including pilots