All but the most veteran of Washington visitors will want to visit the perennial sights of the capital. Here are some guidelines: Tours of the White House until Memorial Day are available on a first-come first-served basis at the East Gate. You must be in line between 10 a.m. and 12 p.m. After Memorial Day, tickets can be obtained at the waiting area on the Ellipse at 8 a.m. for tours between 10 a.m. and 12:30 p.m. All tickets are usually sold by 8:30 or 9 a.m., so get there early to avoid disappointment. As an alternative, you may contact your congressman for tickets one month in advance. For information call (202) 456-2200.
For information about tours of the Capitol call (202) 225-6827. Daily guided tours are given from 9 a.m. to 3:45 p.m. For sure access to the House and Senate chambers, contact your congressman or senator in advance for a pass. For committee hearings, see the local papers for schedules.
''Step-down tours'' of the Washington Monument will be given starting the end of April. These are guided historical walks down from the top of the monument at 10 a.m. and 3 p.m. Washington Monument is open from 8 a.m. to midnight. The Lincoln Memorial is open 24 hours a day. Space is still available in June for ''Under the Memorial'' tours to see the stalagmites and stalactites that have formed underneath. The Jefferson Memorial is also open 24 hours a day. In summer , military band concerts are usually given. For all monuments, call (202) 426- 6700.
For the C&O Canal boat, canoe, and bike rentals at Fletcher's boathouse, call (202) 244-0461. Barge trips begin from either Georgetown or Great Falls Tavern - for reservations call (301) 299-3613.
Mount Vernon is 16 miles south of Washington. There are no guided tours and reservations are not necessary. The house is open seven days a week, from 9 a.m. to 5 p.m. Admission is $4 for adults; $3 for senior citizens; and $1.50 for children. For more information call (703) 780-2000.
A variety of cultural events take place at the John F. Kennedy Center for the Performing Arts. For general information, call (202) 254-3600. A special family event, ''Imagination Celebration 1983,'' runs from March 27 to April 10. One of the events is ''A Bride From Pluto,'' an operatic space fantasy. Tickets are $7 .50 each; call the concert hall box office (202) 254-3776 or Instant Charge (202 ) 857-0900.
A good family guide to Washington, Greenacres school's ''Going Places with Children in Washington,'' ($4.95) is available in D. C. bookstores; or send to 11701 Danville Drive, Rockville, Md 20852.