In Disk Utility you can select the drive on which you wish to erase free space, then after clicking the erase free space button it offers the option of doing various levels of erasing. As I said, most people would find a single write of zeros sufficient. It would likely take a lot of money and a truly professional data recovery service to get anything off a single erase. You can also choose 7 writes, or 35, but these will tie up your drive for quite a while, especially if you have a lot of free space.
Once you do a free space erase once, as long as you use secure empty trash when emptying the trash any future files will also be overwritten.
I am not sure what those utilities posted by another helper offer that Disk Utility doesn't, but I haven't looked at them.
Also consider why you are doing this, and disadvantages. Most people would only need secure erase when selling a computer or disposing of a drive. If you truly have secure data that requires this kind of thing I hope you also have other security measures implemented for the files you do want to keep (forget about your login password - that's easy to get around). Multi writes of this kind will tie up your computer and I have read can be hard on a drive since it is essentially writing to the whole drive in one session. It is also not uncommon that somebody accidentally trashes and empties a file they did not intend to delete. If you secure erase it will truly be gone (happened last week with somebody on one of the forums). While it seems to be a cool feature, I would hesitate to use it unless you really feel the need.
