I want to visit washington dc?

Sweetie is correct that tours are only offered through your Congressman's office. I'll add that four weeks is more like a minimum time in advance to ask--the further ahead you ask, the more likely you are to get passes.

If you do get passes to visit the White House, they are free.

Here's what www.whitehouse.gov says:

Public tours of the White House are available for groups of 10 or more people. Requests must be submitted through one's Member of Congress and are accepted up to six months in advance. These self-guided tours are available from 7:30 a.m. to 12:30 p.m. Tuesday through Saturday (excluding federal holidays), and are scheduled on a first come, first served basis approximately one month in advance of the requested date. We encourage you to submit your request as early as possible since a limited number of tours are available. All White House tours are free of charge. For the most current tour information, please call the 24-hour line at 202-456-7041. Please note that White House tours may be subject to last minute cancellation.

I don't know how much it cost, but you have to write a letter to your state congressman asking him to write a letter to their congressman if you can visit the white houseand it has to be in a group of 10 and you have to write a month in advance