MySQL Replication without stopping master
There are two approaches you can try with no or minimal downtime
Given the following:

Master IP is 10.1.20.30
Slave IP is 10.1.20.40

APPROACH #1 : Data is 100% InnoDB
This is very straightforward.
STEP01) If the Master does not have server-id defined in my.cnf you will have to add it
[mysqld]
server-id=100

STEP02) If the Master does not have log-bin defined in my.cnf you will have to add it
[mysqld]
log-bin=mysql-bin

STEP03) If you have do Steps 1 and/or 2 on the Master, do service mysql restart (mandatory)
STEP04) Create MySQL Replication User on the Master
mysql&gt; GRANT SELECT,REPLICATION USER,REPLICATION CLIENT ON *.*
TO repluser@'10.1.2.30' IDENTIFIED BY 'replpass';

STEP05) Create a mysqldump as a point-in-time snapshot on the Master
MYSQL_CONN=&quot;-uroot -ppassword&quot;
MYSQLDUMP_OPTIONS=&quot;--master-data=1 --single-transaction --flush-privileges&quot;
MYSQLDUMP_OPTIONS=&quot;${MYSQLDUMP_OPTIONS} --routines --triggers --all-databases&quot;
mysqldump ${MYSQL_CONN} ${MYSQLDUMP_OPTIONS} &gt; MySQLData.sql 

When done, line 22 of MySQLData should have the binary log and position of the Master as of the moment the mysqldump was launched. To see it, just run
head -22 MySQLData.sql | tail -1

STEP06) Create replication status on the Slave with
CHANGE MASTER TO
MASTER_HOST='10.1.20.30',
MASTER_PORT=3306,
MASTER_USER='repluser',
MASTER_PASSWORD='replpass',
MASTER_LOG_FILE='mysql-bin.000001',
MASTER_LOG_POS=4;

STEP 07) Load the mysqldump into the Slave
mysql -u... -p... &lt; MySQLData.sql 

Don't worry about replication starting at the right place. Remember, I said line 22 contains the command with correct binary log and position.
STEP 08) Run SHOW SLAVE STATUS\G
If Slave_IO_Running is Yes and Slave_SQL_Running is Yes, CONGRATULATIONS !!!
APPROACH #2 : Data is InnoDB/MyISAM Mix
Rather than reinvent the wheel, please read my earlier posts on using rsync to make a Slave

Jul 08, 2011 - MySQL slave replication reset with no Master Downtime (using MyISAM)
May 23, 2011 - How can I move a database from one server to another?
Apr 08, 2011 - Create a MySQL slave from another slave, but point it at the master

Give it a Try !!!