Classes bloom like flowers in the spring. It's the time to revive those New Year's resolutions that got frozen out in mid-January; the time to start something, do something new. Like flowers, too, there are so many classes you may not know which to pick. Let the warm weather go to your head, for instance, and you could be signing up to learn how to write a book, change a tire or cook for company.

Catholic University is offering a dozen weekend courses starting Saturday and continuing through June 5. By that time you can have written the great American novel if you take the course in writing books, or painted your bathroom ceiling like the Sistine Chapel if you sign up for a painting course. Classes cost from $30 to $50 at the campus at Fourth and Michigan Avenues NE. Call 635-5789.

The Smithsonian is another place for springtime cleaning of the braincells. Weekly classes start April 19 in the arts, humanities, sciences and studio arts, taught at noon, 6 or 8. A few examples: lectures by cartoonists and their editors in "Drawing Board: Cartoon Art" including Jules Feiffer and Jeff MacNelly. Or "The Detective," a course tracing whodunits and their heros and heroines from Miss Marple to Lord Peter Wimsey. Classes in architecture, genetics and photography make up the 90 six-to-eight week courses costing about $70 each. Register two weeks in advance; 357-3030.

Area colleges offer noncredit courses as well. If classes make you fall asleep, don't sign up for Montgomery College's insomnia workshop: Spend the money ($30-$60) on something you really need, like German wine tasting or basic auto. There are foreign-language and art classes starting in late April: 331-3539.

George Mason University offers a cram session though no spring session of noncredit courses: a literary symposium this weekend is free and open to the public. Participants include Susan Sontag, D.M. Thomas and James Merrill, considering the role of tradition in literature. Panel discussions and lectures are Sunday 2:30 to 10 and Monday 10 to 10. Call 323-2138.

The college phone numbers: AMERICAN UNIVERSITY -- 686-2000. GEORGETOWN UNIVERSITY -- 625- 3001. NORTHERN VIRGINIA COMMUNITY COLLEGE -- starts April 12. Annandale campus, 323-3159. Alexandria campus, 323-4212. Loudoun campus, 323-4551. Manassas campus 368-0184, ext. 242. Woodbridge campus, 670-2191. ext. 242. UNIVERSITY OF MARYLAND -- 454- 4712. MONTGOMERY COLLEGE -- starts April 12. For information on all three campuses call 301/279-5188. PRINCE GEORGES COMMUNITY COLLEGE -- 301/336-6000.