The LIS (sometimes also referred to as LIA, Line Information Analyst) generally sits alongside the Controller in a line control room and will assist the controller as necessary, generally passing service information to stations, drivers and other relevant parties when there is disruprion. They also broadcast the service information updates that form the basis of the local PAs on trains and stations.
It's a very good introductory role to service control, particularly for those who might not be as adept at a safety critical role like Service Operator initially.
The best advice is to make arrangements to visit a line control room and shadow an LIS for part or all of a shift, which will give you a better insight into what the role entails. It certainly isn't for everyone.
All the information and practice materials for the SJT will have been sent to you. You should need any more than this, coupled with your existing knowledge of the operational railway and its stakeholders.