That is one broad topic!
If you haven’t already done so, you may want to review Modernizing IBM eServer iSeries Application Data Access – A Roadmap Cornerstone, a redbook IBM put out a few years ago discussing the movement from DDS and traditional data management to SQL.
One of the nice things is that, with some considerations, SQL can work with files defined using DDS and traditional data management (as used by COBOL) can work with tables defined using SQL DDL. So a staged migration is certainly doable.
Hope this helps,