Okay, a few problems here:
1. Go use Google. If you're such a beginner that you can't look it up yourself, there's a good chance you shouldn't be coding as a job just yet. Not to discourage you - just to discourage writing code for important systems before you know exactly what you're doing.
2. Good systems will generally use a database (a way of saving information without manually writing to some file). They also provide a lot of additional things, like increased speed, data integrity, and protection against information loss.
What happens to your program if the power goes out? (You'd lose data if you weren't running a database)
What happens if the CPU overheats and the system off instantly? (You'd lose data if you weren't running a database)
What happens when the listed customer for a transaction suddenly doesn't seem to point to any known customers? (With a database, you have data integrity)
Though to answer your actual question, try this: Google Search for visual basic file i/o