You need the entity wizard to create a new entity 

Norman

Does your company have a Sage partner?  They should be able to step you through the creation on a new entity.  But making extensions to the system needs to be considered carefully and planned for appropriately.  

Do you have a development and test environment in place?  You should never play with the live system.

Vega has mentioned the Advanced Customization Wizard that will create the screens (and potentially the ASP pages) that automate so of the process for you.  But you need to think about the API that you will use.  Will it be ASP pages or would your prefer to use the .NET API?

Have you established how you are going to change the datamodel?  Do you only need to add a single 'primary' entity or do you need to add secondary entities?

Have a look at the eLearning resources - the developer ones are arranged in a 'hierarchy' of learning https://community.sagecrm.com/e-learning/default.aspx

I am working on a non live system, so no worries there at the moment . We do have a Sage partner but they are not available all the time.

I've gone ahead and created the new entity using the Advanced Customization Wizard and following the corresponding tutorial video. So far it seems to have done exactly what I wanted it to.

Good.  Please do take database backups before making changes to the data structures.