The first step for any strategic APM initiative is to examine the IT Service Catalog, assuming it exists, and perform a Risk Analysis to determine which apps to look at more carefully.
Risk Analysis involves rating the Likelihood and Impact of availability and performance issues to see which show up as High risk.
These apps should then be explored in greater detail in terms of business context, cost of poor performance/downtime, number and location of users, and technology and monitoring environment.
What should also be considered is what KPIs exist, or are desired (and will need to be instrumented) on availability and performance, and how these link to business metrics. Then any APM initiative will have a “shopping list” for what visibility needs to be created.
Of course, there is always the alternative, “just start spending money”….