Hi lifezz,
Before starting any customizations you need to have drawn out your plan so you know what the process flow would be from beginning to end for your project, you can either do this on your own or you can speak with one of our Consultants. It's very important to plan out customization work properly because you'll need to have the blueprints to how everything would work. The forums are here for guidance regarding particular processes not entire projects; for example, you asked how you can create an RMA section under the customer portal, this can be done by creating a new screen (Customize > Screens) then adding a tab link for it under the Contacts Portal tab.
If you require assistance with planning out your project or if you want someone to perform the customization for you, please let us know and we can get one of our Consultants to reach out to you.
- Ashur