What defines official Christian belief? Almost every Christian I meet believes in something a bit different.

Is there an official set of things you are supposed to believe in order to be considered a Christian? If so, who decides what goes on that list list and what doesn't?
Christianity is not a single group, it is a diverse set of beliefs.  Even though it originated with the teachings of Jesus back 2000 years ago, difference sects/denominations have their own sets of rules and "official" teachings and many individuals determine the rules for themselves.  There is no global trademark owner of "Christian" so you can call whatever it is you believe Christian and though other Christians may disagree and not form a church group with you, in the end God is the only arbiter of that, and he tends to not send rulings via email.

Some churches, like the Catholic church, have a large body of teachings they consider "official" and will hassle you more or less officially if you deviate from them/teach deviations from them.  Other churches stress personal interpretation of the Bible and therefore don't have official teachings (though often have unofficial ones, that you may be ostracized from deviating from).

There are certain sets of belief that large swaths of these groups consider to be normative.  For example, many mainline churches consider the Apostle's Creed to be the minimal acceptable set of Christian beliefs. Outside that, they'd say that you're using the brand name, but probably inappropriately. But even the creeds like this leave a whole lot out; it's a couple of paragraphs compared to a whole belief system.

This is augmented by a lot of folks not really having any rigorous thought process behind their own personal theology - when asked a question they just come up with an analogy that sounds good to them at the time, whether their church would say, upon sober review, that it was accurate or not.  So if you ask a variety of folks self-identifying as Christians about stuff, you are likely to get a variety of answers - there will be more consensus closer to core matters of the faith, and from people who belong to a church that has a stronger stance on having a corpus of official teaching, but in the end it's a big batch of folks with a big set of beliefs, guided by a lot of group and personal history. 

As a result this SE says "hey, if you self identify as a Christian, you're in scope here," even though some of those groups don't fit into some of the traditional creed sorts of definitions, because in the end no one knows for sure who was right till we get to heaven...