'Prufrock' is an extremely complex poem synthesizing many of what Eliot considered important ideas about history, poetry, and the personal. As Eliot himself points out in his seminal essay, 'Tradition and the Individual Talent', poetry at its most profound is the tradition of culture as a whole mediated through an individual sensibility. Thus in 'Prufrock' we hear voices of Shakespeare, Hesiod, Renaissance art, Matthew Arnold, etc.
The first major theme is that of indecision. Prufrock, like 'Hamlet', is a man who cannot make up his mind -- but unlike Hamlet, this is applied to the personal choice of his relationship with a woman rather than to the fate of a kingdom.
Another theme is modernity, and the way in which it dislocates the self from place, religion, and tradition, not only making the individual isolated and atomistic, separate from land and community, but also fragments the individual himself, separating emotion from intellect, reason from religion, etc.