Every philosopher with a webpage links to Jim Pryor’s guide to writing a philosophy paper, because it is an outstanding resource.
Wait! Don’t go to wikipedia! Try the Stanford Encyclopedia of Philosophy instead. It has clear, comprehensive, rigorous overviews of nearly all major philosophical topics and figures.
Everyone who teaches you writing will point you to Strunk and White. Read Chapter II. But also beware.
In an attempt to make an original contribution to your success as a writer (of philosophy or anything else), let me point you to Garner’s Modern American Usage. It is an exhaustive resource. Many graduate students haven’t yet discovered it, and for that they are worse off. If you’re jaded by the Oxford Comma wars, take a look at Garner’s four-page entry on phrasal adjectives.