2000 Elantra

my friend here has the 2k elantra and it's having some serious engine trouble right now. for one, its engine idle in gear is really low(300rpms) and the check engine light is on when you start it. it also doesn't even go above 20mph(while floored) at idle, it shakes really bad and feels like its missing all the time. i hope it's only spark plugs, but any help would be appreciated