Your question is broad and open ended, probably by design.
The good thing is that using the search function on this site will reveal the many answers that many others in your position have received.
Reading some of the diesel coach issues that have been posted and resolutions for those issues will provide you with a wealth of information to assist in your learning curve.
Using the history contained in this forum can be priceless!
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs