Various online services help people find their birth mothers for free. They include Iwasadopted and Genesreunited among others. One may however be required to register so as to get a free account.
1 Additional Answer
The first step into finding your birth mother is to establish whether she is alive. Once you confirm that she is alive, start by asking your father (if he is alive) and relatives as much information as you can about her. Using the information that you have, register at genesreunited.co.uk, a website that will help you trace your mother for free.