Not sure how much you know, so I'm assuming not much...
Once you have a guitar that is comfortably playable and holds its tune (two essentials), don't skimp on the amp. After decades of using very ordinary amps (mostly a fender super 60), and thinking it was ok, I finally bought a handwired vox AC15 and fell in love with playing all over again. Assuming you're going electric I'd also recommend a good headphone amp like a "vox amplug" or similar so you can practice without anoying anyone.
I had a few lessons when i started at 18 and taught myself after that, practicing chords and scales. Learn as many chords up and down the neck as you can, and practice scales (pentatonic is by far the most common in rock, blues etc). I think what I really could have used but never had was a good mentor - try to find one if you can.
I have a few guitars and like them all at different times depending on my mood and desired tone:
- Early 80s Telecaster usa (bought new, needs new frets now)
- Gibson Nighthawk 1994 (very versatile, more than just a 'poor man's Les Paul', but half the price)
- Maton MS2000 (Maton being Australia's only large volume producer, often overlooked)
- Fender Catalina steel string acoustic (my first guitar)
- 70s yamaha classical
Also, think about what guage strings suit you best for your guitars - light strings are easier to play but you will lose tone. Heavier strings don't mean you can't play fast providing you have strong enough fingers - I've read that Stevie Ray Vaughan used 13 to 50-something strings, while most players use much, much lighter ones (I like 11-49s on electrics).
Great thread btw.