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You asked about advice about a 12 string. If you noticed the list of related threads at the top of this one, you can get a whole encyclopedia of knowledge if you scroll through them. One word of special advice, any guitar store will charge more for a 12 string than a 6 string. The problem is, you can pay a little more and get a real piece of junk, or you can pay a little more than that and get a quality instrument. The twelve strings add a considerable amount of tension on the bridge, and if it is not braced correctly the plate will pull away from the top. Also, the neck has more tension on it so it tends to warp more. Some of this can be compensated by lowering the tuning (hence the many discussions on 12 string tuning) but a really well made instrument should hold standard tuning. Now, about price. If you are serious about a nice (new) guitar that will last a few years, I would say $800 - 1,000 would be a minimum. There are probably some screaming deals out there lower than that, but buyer beware. I personally would start at about $1,200 - 1,500, and from a reputable company or luthier you know and trust. Used prices will be somewhat less, but make sure the bridge is in good condition and that the action (the distance between the strings and the frets) is acceptable to you. As far as tuning, on the lower 4 courses (sets) of strings, each string in the set is tuned an octave apart, in the two highest courses each string in each set is tuned to the same note. So what you would have in standard tuning is eE, aA, dD, gG, BB, EE. That is what gives the 12 its distinctive sound. Then, there are a multitude of alternate tunings, and you can play with the octaves as well.
Hope you get your 12. It is almost a different instrument than your 6 string, and you will learn to love both of them for different reasons, and use them for different purposes. I had a 12 as a teenager and gave it up, to much moaning and groaning later. Now I have another one and I will give it up when they pry it from my cold dead fingers (to quote a well worn out phrase).