"Sich" is used for reflexive actions, i.e. when the subject and object of an action are the same, in the 3rd person:
– He washes himself. = Er wäscht sich.
– They wash themselves. = Sie waschen sich.
Likewise for prepositions:
– He talks about himself. = Er spricht über sich.
The pronoun systems in German and English are slightly different there. In English you always have a couple "X"/"X-self" of object pronouns for non-reflexive and reflexive use, in German the pronouns only differ in the 3rd person. For example:
– He washes ME. / I wash MYSELF. = Er wäscht MICH. / Ich wasche MICH.
– I wash HIM. / He washes HIMSELF. = Ich wasche IHN. / Er wäscht SICH.
"Selbst" is used (for all persons, not just 3rd) to emphasize that someone does something oneself (but not *to* oneself as above), personally, on one's own:
– I'm coming myself. = Ich komme selbst.
– He writes the letter himself. = Er schreibt den Brief selbst.