you can just type / for a template, or use these links for pre-defined diagrams:

https://app.gleek.io/diagrams/basic https://app.gleek.io/diagrams/sequence https://app.gleek.io/diagrams/erdiagram https://app.gleek.io/diagrams/class

there is also syntax help link in the bottom-left corner

your feedback is great, we should make it more clear on how to start