Well you should store it in a database, most probably an SQL based database. MySQL is a fine start, free and easy to set up.

But, you had best start thinking about security issues, especially if you are planning on storing credit card and other personal information. Hackers go right through the standard fare and many companies do nothing to encrypt their data at all, which in some countries is a criminal offense.

Best hire a professional to look at your system setup and see what can be done.