I'm impressed with how some people can tell the difference between breeds when they look so similar (to me). I don't think I could tell between a white rock, white cockerel, and a white leghorn.
So... here's a little primer on the difference between these breeds. All have yellow legs. They have different body shapes too, but with hatchery birds these aren't as obvious, and it's harder to distinguish if you're new. But the comb and earlobes are obvious.
White rock - red earlobes, single comb (like yours pictured)
Rhode Island White - red earlobes, rose comb
White leghorn - white earlobes, single comb, also much lighter in body overall than the other two, a smaller bird
This one is a cockerel because at 12 weeks, a pullet would not have that much comb and especially not so red.
I'm now to this too so I've been trying to learn what to look for and thought I'd share!