How to get started/pick/understand what web service tools to use

How to get started/pick/understand what web service tools to use

Ok, so this is an open-ended question. I would like to start with I am not
looking for an exact answer, as much as I am trying to find the correct
path to educating myself for what I need to do.
I am an iOS developer (5 years now) and I also know a decent amount about
HTML, CSS, and some PHP. I have dabbled in JavaScript and barely even
touched Ruby (On Rails), so it is fair to say that I am a novice web
developer. I can build great front ends but the backend and connecting it
are where I need help.
I am looking to build a product that has a few requirements. Here they are:
Scalable database to store ever increasing data over time. Users will be
continually adding new content that will need to be stored and also
indexed to be viewed later. This will include text, photos, and possibly
video clips (most likely not initially).
The ability to handle messaging. This is not a chat service, but rather
the ability to click on a user and say send message, and they will have a
dashboard section that will allow then to view and respond to those
messages. This will also be stored on the database. Nothing profound here,
standard implementation.
Easy access to maintain this data and content. I will need to be able to
quickly query the submitted content and grab all the data for users to
see.
Obviously the need to manage user accounts. Seems like a separate table or
DB for user accounts and linking them with content is needed. I feel like
this and number 3 are subsets of number 1 so I apologize if this is
redundant.
The bottom line is there are so many options, I have no idea what will or
will not work for my site. I barely know command line and terminal, I know
nothing about servers and Amazon Web Services seems like an amazing tool
but it is filled with tons of technical stuff I don't understand.
I am the type of person who will read books, watch videos, read tutorials
and constantly scan over Stack Overflow to learn new things, but in this
instance I am not sure what I should be learning.
Any idea's on what programming language I should be using? What web
service will fulfill my requirements? How will this all tie in to the HTML
side of things?
I am looking for some guidance on where to start, and ANY information will
be helpful. Links to articles to read, suggestions, personal experiences
are all welcome.
Thanks!