Development newbie - Unsure of which language to learn

Hello.
I'm new to the forum, and new to developing, too. I've been a frontend designer and graphic designer for about 5 years, and am looking to learn some new skills, and get in to development for web, and mobile applications. The problem I'm having is I'm a little confused as to which language is best for the kinds of things I'm looking to learn how to do.

I'm comfortable with HTML, CSS & graphics work, but think that my work is held back by the fact that I'm reliant on open source CMS platforms/e-commerce platforms to be the 'backbone' system behind my website, as I lack the Javascript/PHP/Ajax knowledge needed to create dynamic pages.

I also find static websites (just simple HTML, CSS and the odd bit of jquery) are not practical because clients wish to edit their own sites after they go live. This also impacts on my design too, because the look and contents of the websites I make are kind of dictated to by which Wordpress theme I use and the features that come with it.

Basically, I'm looking to be able to do the following things (as examples):
1. Code and create contact forms, newsletter sign-up forms and event registration forms for websites.
2. Code and create bespoke content management systems and/or e-commerce platforms.
3. Create mobile apps.

From what I understand, the Android apps are written in a different language to iPhone ones, so I understand that there are limitations and benefits to each language, but I'm just not sure which one would be best to begin learning to do the things listed above.

Thank you in advance for any help and advice, I'm looking forward to learning a lot of new things from the forum and appreciate any help given

DanC