Make an effort on one's own behalf. Shakespeare used this expression in 2 Henry IV (3:2): “She is old, and cannot help herself,” and it also appears in the old proverb, God (or heaven) helps those who help themselves. [First half of 1500s] Also see can't help.
Serve oneself, as in The food's in the kitchen; just help yourself. When it takes an object this phrase is put as help oneself to, as in I helped myself to more meat. It also is used as a euphemism for stealing, as in She simply helped herself to the hotel towels and left. The first usage dates from the late 1600s; the second, a colloquialism, from the mid-1800s.