The first, most critical, thing you can do is start reading about office politics. A supervisor who is inept at politics harms the people working for him as well as himself. You need to understand politics to get your people the raises they deserve, to keep the projects on track, etc. You need to start building organizational allies.
Some reading on communication styles is helpful as well. Deborah Tannen has some good books on this subject.
Getting to know the people on the business side that you are developing products for is very helpful. You need to balance their needs with the team's ability to fulfill them, so you really need to understand where they are coming from. If your users are outside the company, you need to find a way to connect with those people as well to start to understand their perspective.
Some technical conferences also have soft skills training. Wouldn't hurt to attend one of those and attend some of teh soft skills sessions.
Not sure how to prep you for making difficult decisions that affect the lives of people who work for you. All I can say is that through the years, I have noticed that this is one of the big make or break things about a good supervisor and that putting off a negative decision tends to make it worse in the long run. Telling the client that you won't meet the deadline is much worse after you have missed it than weeks ahead. Telling someone they are being fired is much worse that telling them they have a performance issue and exactly what they need to overcome it and then helping them do so.
I have noticed that a team will react better to negative news from a supervisor they trust to look out for them with the people above their group.
It would also be helpful to get with your HR department and get training on the processes you need to know about as a supervisor including such things as time reporting, dealing with performance problems, how to do performance evaluations and set annual goals, etc. Know the process for dealing with a performance problem for instance can help keep you from doing the wrong thing before you ask for HRs help.