Call.

For software: what the current system is, who uses it, and what it does that nothing else can. If you know where the source code is, that's a bonus, but most people don't.

For a website: your hours, your address, a phone number you actually answer, and what you want a visitor to do. Photos and a logo if you have them; it works without.

If you're an IT provider calling about a client: just the outline. I'll sign whatever you need before hearing the details.