I believe DI would help

I expect it's more than mere "help", it's the key

not quite sure where to start

Have you read MSDN's DI article? Use .NET provided classes to bundle your interfaces and base classes into 'service containers.' These service providers instantiate concretions that are then passed into the using class' constructor. Reading this feels at bit like using a parts catalog to scratch-build a car; maybe this book can help: Dependency Injection in .NET

I believe DI would help

I expect it's more than mere "help", it's the key. But you already use DI if using parameters in constructors and other methods because that's all DI is. Fortunately .NET provides classes for dynamically DI-ing large, complex sets of dependency code.

not quite sure where to start

Have you read MSDN's DI article? Use .NET provided classes to bundle your interfaces and base classes into 'service containers.' These service providers instantiate concretions that are then passed into the using class' constructor. Reading this feels at bit like using a parts catalog to scratch-build a car; this book will help: Dependency Injection in .NET