The following documents represent a variety of perspectives on social activism, with an emphasis on sex-positive activism.
To learn the basics of activism and citizenship, however, your best starting place is probably the book The One-Hour Activist: The 15 Most Powerful Actions You Can Take to Fight for the Issues and Candidates You Care About. In fact, if all you want is to make a difference as an individual, you can probably stop there. But if you're also interested in working with organizations, then you may need to graduate to The Activist's Handbook: A Primer if you're part of a new organization, or The Citizen's Guide to Lobbying Congress if you're in the position of being part of a national organization directly lobbying Congress.
Note that these books, though they teach essential skills, do not offer implementation and rhetoric suggestions specific to sex-positive and first ammendment activism. For that piece of the puzzle your best bets are all free publications available online, e.g. the "Key Issues" on the Woodhull Freedom Foundation web site, some of the documents in the "Resource Library" section of the National Coalition for Sexual Freedom web site, and the individual "Issues" sections of the ACLU web site.
If you're new to this site, we recommend you visit its home page for a better sense of all it has to offer.