You need to use Idocs. That’s SAP’s equivalent of EDI. You cannot use BAPIs or RFCs to create an EDI document although the Idocs do occasionally make use of them. Another alternative is ALE which also uses Idocs to send data to remote systems like your vendors, though this is more usually used I think in SAP to SAP set-ups.
Part of what you need to do is identify the (EDI) standard used by your vendors’ systems. One one of the more commonly used standard is the EDIFACT standard. That controls how your EDI document will be formatted.
Then you need to configure your Idoc set-up. That’s includes setting up your Destinations (e.g. your vendors’ systems). Your Basis consultant should know ow to do this. Other things you need to set-up are you Partner Profile and Port Definition. You also need to specify the Idoc types / message type you’re going to process.