normally you would design the database schema, if you have existing data in another db or other electronic source you can do a bulk load of data. Usually you would only do this on the intial set up of the system, or if the system was a client consumer of data form another / other datastores (eg if the db ran locally and used data from a master db located elsewhere). examples of local systems could be say a university which has a master db controlling student admissions / enrollment and a local (say faculty) db whihc handled things that are unique for that faculty.
tools to do that exist MySQL will allow you to load data from a file, PHPMyAdmin has a usefull tool as has MySQL Administrator (both of these can take data either in or out using ASCII / text files). There are other tools such as MySQL Migration toolkit & dbTools which will transfer data from one db to another.
once the DB is up and running you would normally use SQL to manipulate the data in the DB, to either INSERT [add], SELECT [retrieve], UPDATE [change] or DELETE [does what it says on the tin] records.
so in the normal world you would have a client applcation written in what ever language you like (eg PHP/ASP etc in the web world, C,C++,
VB, .NET etc) in the conventional world which provide the functionality you want, whether that be to add new, change exisitng, delete old records, or merely to view whats there.
if you dont have many records to change or delete then theoretically you could do it using MySQL Query Browser, however its fine for a simple quick and dirty SQL Id not want to use soemthing like that in a live / real db.