My biggest piece of advice I give to every new dental student and resident in my program is this - DO NOT put the dam on until you're ready to put files into the tooth. There is no reason to have the dam on prior to that, the pulp is already infected, a bit of saliva won't cause much harm at this point. It will help you immensely with your access to be able to better visualize the tooth.
Next, I totally agree about doing caries excavation first, as yak said, it will often lead you to a pulp horn, and then you can expand the access from there. One other thing I see on people who are doing their first few root canals is that they think they're in the canal, and have unroofed the chamber, but actually have just opened up a pulp horn. On molars, you can usually read a "map" on the floor of the chamber, with lines leading to each orifice. An orifice won't just look like a sudden rough hole, that usually means it's just a pulp horn.
Lastly, the MOST IMPORTANT piece of advice - when in doubt, take a bitewing. If you're concerned you're not going in the right direction for the access, or you feel like you should be there already, just stop and take an X-ray. Taking a bitewing is quick, perf repair isn't! 😉
Good luck!