THIS IS A GREAT SITE:
http://curious.astro.cornell.edu/cosmology.php
It is an "ask an astronomer" site suited for bright junior high and high school kids.
Scroll down the page and you will find a list of questions that young people wrote in asking and which the Astro/Cosmo graduate students at Cornell answered.
Whatever question catches your attention you can click on it and see what the Cornell people answered. they also have lists of most popular questions.
I WOULD ADVISE YOU TO START MORE BROADLY AND READ Q/A IN OTHER ASTRO AREAS
because your understanding of Cosmo will be much better if you understand things like star-formation and how supernovas work and how people measure distances in our own Milkyway galaxy (much smaller than the observable universe as a whole :-D).
So I would say, if you haven't done it already, to start here on the main homepage:
http://curious.astro.cornell.edu/index.php
And look over on the lefthand side where the Q/A menu is and look down to where it says
THE UNIVERSE
Extrasolar Planets
Stars
Supernovae
Black Holes
And Quasars
The Milky Way
Galaxies
Cosmology And The Big Bang
Each one of those 7 topics is a link to a list of Questions that have been asked (by young people like you) and that have been answered by older students working on their PhDs.
I would learn about ALL those things, not just about Cosmology (the last on the list).
After you have tried that, come back here for more advice
I don't agree with every answer I've seen given at the Cornell Q/A site because I think they sometimes oversimplify and sacrifice part of the story to make it seem easy. But on the whole it is an excellent resource for smart motivated young learners.