Allowing every person on the planet with internet access to create new resources on your site is kind of asking for trouble.

Imagine that a bot fills in and submits your contact form a few thousand times.
Can you explain why you want to do that? Maybe there's another way.
Calling the 'resource/create' processor requires that the user have at least new_document permission (and publish_document if the doc is published). In either case, they have to be logged in.
If you really want to do it, though, $modx->newObject() is really the best way, though you're going to want to sanitize the crap out of the user's input before letting it get anywhere near MODX or the database.