The Support Centre has many pages with contact points on them.
Send in a message via a contact point and you will get through to a human JMod.
Due to the volume of messages, it can take anything from a few hours to a week to get a reply.
It really depends what you want to ask about.
If you want to start here with a basic description, we can point you in a more specific direction.
Sometimes it can be hard to get through about certain topics. Jagex do have some restrictions about what they are prepared to discuss.