The first step would be to figure out where sendmail is installed. Once you know that path, go into your php.ini. You are looking for the sendmail_path setting. Set it appropriately.
If the path is set correctly, the result from a proper call to mail() should return true. (Note that the return value only lets you know if the message was passed on to sendmail [or SMTP in the case of Windows]. It does not guarantee that the e-mail went out, or that sendmail is configured correctly.) If it is returning true and you still aren't getting e-mail, then check your sendmail configuration.
