You've been dropped in an environment that is legacy and probably has production problems. Use that to your advantage.
You've been also dropped in a leadership role (not management, leadership).
Your #1 target should be to make yourself redundant (which ironically is likely to get you promoted, it's called succession :).
So look at doing something like identifying #1 problem (Pareto charts help). Ask for volunteers (or volunteer some people), give them the problem to solve, use whiteboards, etc to help them discover the solution. You may facilitate and provide hints to get things done. Empower and guide the people you are helping.
Read up on https://en.wikipedia.org/wiki/..., you are likely in a #2 or #3 combination. You can help lead people to move to a #3 with leadership, with the idea to get to #1 over time (with their help).
Of course there might be some issues that you might need to solve like EOL systems and any budget that may be needed. If the OS is old, then probably the HW is old as well. Budget for that is probably going to be your biggest issue.