Don't know what you need? Then chances are you lack the performance and capacity requirements necessary to write a useful RFP, not to mention the knowledge necessary to evaluate the responses. In this case, consider avoiding the RFP process altogether and issue an RFI (request for information). In so doing, you may avoid useless proposals from drive-by companies who aren't really that interested in whether what you buy actually suits your needs.
Instead, an RFI will garner responses either directly from hardware vendors (if you're large enough) or resellers who will consult with you to help you determine your needs. This is often a great way of getting free consulting services if you don't have the time or resources to do capacity and performance analysis in-house (though beware that you may get what you pay for -- obviously biased advice is sometimes less useful than no advice at all).
Set the stage with an overview of IT operations
The first thing a good RFP will do is paint the landscape. Many companies responding to your RFP may know nothing about your environment. Describing your overall IT operation from the 50,000-foot view, before you dive into the specifics of exactly what you're looking for, lets respondents calculate how their solution may fit in with the rest of your environment.
For example, say you operate a mainframe environment alongside a virtualization environment that primarily hosts Windows servers. Even if you're just looking for new storage resources for the virtualization environment, describe the mainframe environment also. You may find that some storage solutions you implement for one could work for both in the future. Even if you don't want to change anything now, having that flexibility may save you a pile of money down the road.
Don't omit the human factor from your overview. If your staff has never operated a SAN before, they'll need training to get up to speed with any new solution. Likewise, if your staff has only ever used a specific type of SAN (Fibre Channel vs. iSCSI, for example), vendors proposing a different technology will know to include the necessary training in their proposal. This gives you a better idea of what the total cost of implementation will be and makes the proposals easier to compare.
Dive in to the technical requirements
After you've set the stage, it's time to get into the nitty-gritty. This is probably the most difficult part of the RFP. You need to keep your respondents honest and avoid limiting the creativity of the proposals you get at the same time.
If you're seeking a primary storage solution, at a minimum you'll want to define how much usable capacity you need and what your performance requirements are in terms of IOPS (I/Os per second). Combined, those two metrics are your best way of weeding out the multitude of low-end solutions that may not perform well in your environment.