I bought a 12-str. not too long ago, and today I changed strings for the first time.

I started with the Low E's and what the hell, the higher one makes a buzz on the 4 four frets.

It did not before I changed the strings

What did I do wrong?
since it's first time I'm changing strings, I don't know what is the gauge of the other strings. But the gauge I use is 0.10, the regular one
the gauge of the strings really depends on your preference.. thinner strings with a more lively sound, or thicker strings with a more full bodied, deep sound.. and to address DYLANMANCE's
question, a lot of my friends play classical guitars, and the bronze strings all have the weird sound.. id say go with phosphor-bronze. bronze strings themselves are a little too elastic for my tastes.
The buzzing could be attributed to a slightly skewed bridge, or maybe you NEED to go with a lower gauge.