As for a first guitar, yeah, just get whatever really. I played a SYNSONICS guitar $50 for 7 years!! It does not stay in tune long, harder to press the strings, doesn't sound nearly as good, but all the proper tones and what not are there. I hooked it up to a Boss Metal Zone Mt-2 and a cheap amp to practice metal.
The day you decide that you are CERTAIN you want to continue along this path is the day you get a real guitar. If you are really into Black Metal, as I am, then I suggest a Jackson RR-5 ($1,500) or the like in the Jackson line; there are cheaper ones and even more expensive ones. Eventually I might get the $2,400 shredder, but I really do not need it. I am currently recording a black metal album that has the exact sound I was aiming for (high speed melodic tremolo of the early to mid '90s Norwegian scene).
Scales are your best friend and are easy to learn on your own, but even that is not necessary. I am self taught (for 11 years), never knew what a scale was until my fifth year, and by then, I did not even need them. I know where all the notes are by instinct and the sounds that they make just by playing them so often, and the beauty of the black metal style is that it can easily be transcribed to your guitar from your own head without massive chord construction, as many of the notes are obviously single notes at a time, recording track to track. The Metal Zone pedal is a must for a black metal feel, and I suggest also getting a cheap 4 track recorder when you can, unless you are going to be playing with others, as the melodic style demands that you hear what you are creating.
All joking aside though if you're into into black metal learn you Harmonic, Natural, and Melodic Minor scales.
Indeed. Harmonic minor I play in very often. Also, look up tabs whenever you can when you need new ideas for ways of playing your particular style. It also helps to run searches now and then of forums by using google; you will find many of your black metal questions answered in this manner....though I must say, there are few if any sites devoted to teaching how to play black metal, which is a completely different style than most other types of metal (unless it be black metal of the now, which is largely power chords).