There are some questions to consider the answers to:
- What kind of data am I storing? Is it just text and numbers, or do I have to store files or picture? Do I have to store documents?
- How much data am I storing? Do I have a few dozen customers and a couple of suppliers, or do I have hundreds of both? Do I have historical information I need to put in, and how much of it do I have?
- How many people need to get at it? Is this just for me, or do other people need to get at it also?
- What other requirements do I have? Speed, costs, etc. Does the system have to lightning quick or can you wait a second or two? Do you have a budget to spend on this, both for software and somebody to handle it?
- If the amount of data is small, simple and textual you could get by with a text file. An XML file can easily hold a fair amount of structured data and does not require very powerful software.
- If the amount of data is larger and maybe a bit harder to manage you might need a spreadsheet like Open Office Calc, Google Doc or Microsoft Excel.
- If your data needs are larger you start to get into actual "database" software, of which there are many different options from free to very expensive. The choices stretch out a great distance, but include smaller ones like Microsoft Access or FileMaker or ones that will scale out as needed like MySQL, PostreSQL or Microsoft SQL Server (various editions).