I'm assuming you want that oldschool sort of muscular big exaggerated look, since modern superhero artists are generally trending a bit more naturalistic, particularly Marvel, where the look of the heroes is cleaving a bit closer to their appearance in the movies, with impressive, but only slightly exaggerated physiques.
One of the first key things is to study anatomy, because you have to draw a lot of muscles to make this work, and have at least the main ones in roughly the right places.
You'll also want to keep in mind that Superheroes have a "heroic" sort of head:body ratio, as in, while an average real person's total height is about 7 and a half times their head height, a superhero will usually be more like 8 or 9 times, so they tend to have long limbs, a big body and a small head. It's sometimes tough to transition to superheroes for manga artists, because while manga characters have long limbs that make their height comparable to heroic proportions, they tend to be proportioned with quite large heads.
You ideally want the muscles to give the hero a really heroic silhouette, so make sure you do a really broad chest, narrow waist and hips, then good visible contours to the thigh and calves, plus defined shoulder, bicep, tricep and forearms for a chiselled, powerful look to the arms. Don't go too wild on the neck and shoulder muscles though to the point where the neck is lost unless you want a "hulking" look. Keep the shoulders and neck to a fairly nice smooth line for a touch of heroic elegance.
Give the muscles plenty of definition and don't be scared to be really bold with your shadows.
I find that some of the best references for the superhero physique are professional swimmers, gymnasts and rock climbers. They tend to have really well distributed muscle with comparatively little bodyfat, but they're not excessively musclebound and hulky like a bodybuilder.