how do I get good at team building?

the only format i ever got close to the ladder was random battles where i dont have to worry about it.

right now in OU it feels like wether or not I was lucky to bring the right pokemon for certain threats. and whenever I thought I made a change for the better, theres a different threat I no longer have an answer for.

my last team was
muk -a
skarmory
rotom W
lele
excadrill
mega zard x

all pretty much the standard sets