Cite the sources both of passages you quote verbatim, and those you paraphrase. Paraphrase or incorporate as much as possible -- quote verbatim very sparingly. Make it very clear when you are using someone else's work. Choose a form of citation and stick with it -- be consistent.
... the behavior of the Greeks at this time has been compared to that of "fractious, ill-mannered children" (Green 1990, 288). This assessment ...
- or -
... Peter Green compares the behavior of the Greeks at this time to that of "fractious, ill-mannered children" (1990, 288). This assessment ...
Bibliography contains this entry:
Green, P. 1990. Alexander to Actium. London: Thames and Hudson.
Your bibliography formats can be different, as long as they include the relevant information and are consistent.
... the behavior of the Greeks at this time may be compared to that of "fractious, ill-mannered children."1
1 So Peter Green, Alexander to Actium, (London: Thames and Hudson, 1990), 288, in a somewhat extreme formulation.
Be consistent; if you do not feel comfortable with standard abbreviations (in Liddell-Scott and Strunk-White), use the authors' full names. Name the work and the translator, and make sure to include your source in the bibliography.
As a captive slave slips away
from a rich house, whom fate has recently deprived
of her native land, [ ...]
so the lovely girl
rushed forth from the palace. For her the bolts of the door gave way
themselves, leaping back at the shrill strains of her magic song.
(Apollonius of Rhodes, Argonautika IV, trans. B. Fowler)
If you quote a long passage (4 lines or longer), use a block quote -- no quotation marks, and indent the block on both sides.
If you use the historical present ("Hellenistic houses are ..."), do so throughout. Note that the sources you quote may use a different tense; remember to return to the one you have chosen.
No-one is. Neither .. nor ... is. The number of houses is. The ages of man are.
"At the age of three, Green states that Alexander the Great ..." means that Peter Green was three years old when he said it.
"Having been destroyed by an earthquake, the inhabitants of the town began to rebuild" means that the inhabitants were destroyed.
"While walking along the broad avenues, the city made a big impression on her" means that the city was doing the walking.
These are only the most common pitfalls; use the writing center to help you identify any problams you may have. Lay your writing aside for a few days and look at it again with fresh eyes. Have a friend go over it with you. Use all the resources at your disposal to grow as a writer!