I have installed my wamp server in D: drive so u have to go to the following path from ur command line->(and if u have installed ur wamp in c: drive then just replace the d: wtih c: here)
D:\>cd wamp
D:\wamp>cd bin
D:\wamp\bin>cd mysql
D:\wamp\bin\mysql>cd mysql5.5.8 (whatever ur verserion will be displayed here use keyboard Tab button and select the currently working mysql version on your server if you have more than one mysql versions)
D:\wamp\bin\mysql\mysql5.5.8>cd bin
D:\wamp\bin\mysql\mysql5.5.8\bin>mysqldump -u root -p password db_name > "d:\backupfile.sql"
here root is user of my phpmyadmin password is the password for phpmyadmin so if u haven't set any password for root just nothing type at that place, db_name is the database (for which database u r taking the backup) ,backupfile.sql is the file in which u want ur backup of ur database and u can also change the backup file location(d:\backupfile.sql) from to any other place on your computer