This is a very broad question to answer in simple terms. Need more details.
What are you migrating from? What system is in place now? And what systems are on the current system? What language? Etc.
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If you have Office loaded first and then load in System i Access for Windows and you choose to include the Excel Add-in then you can use Excel to upload the data to your IBM. The most difficult item is creating a table for your Excel data to be imported into. This will allow you to get data into the IBM i.
The second tool is SQL using this to convert the data to CSV data type. Excel can read and convert CSV files. Finally SQL on the IBM i, well nothing more needs to be said.