It would depend on where the dent is, if it's on a straight section that can be accessed, just slide the tube over the a mandrel small enough and tap it with a leather or rubber tipped mallet. If the dent is on a curved section you will probably need to take it to a shop where they have the proper tools.
Raw brass polished and clear coated can be very nice for awhile, silver plating (done in a shop that specializes in that operation) will last a long time if taken care of.
I tried for 7 years to Teach Jr High school students to solder, and I would recommend that if you want to learn to solder, do so, but take your horn to a shop and have it done correctly, there are so many things that you could do wrong, it's just not worth it. Teaching yourself will result in a lot of bad habits that will cause more trouble that it is worth. If you don't solder now, by the time you really learn how to do it properly and adequately, your horn could be corroded to nothing. I don't care what others will try to tell you, soldering is not as easy and simple as some will say. Anyone can solder to pieces of metal together and make them stick, but doing the job neatly and efficiently takes years of practice and learning.