To try to answer this all in one go,
-Yes, get a teacher. Before you can read Shakespeare you should read beginning books; similar to piano. Before you can tackle things by yourself, you need to work with someone who has all the experience, that is to say, teaching, performance, preferably coming from a reputable music school (though there are still several good teachers who never went to college). But the bottom line for a teacher is
just because they can play well does not mean they are a good teacher, they need to be able to convey their experience to you effectively.
-Yes, learning by yourself can give you bad technique. To select a good teacher, you may want to see if you can sit in on a lesson. See what the teacher does and doesn't do. See their standards. Also important; have a good relationship with your teacher, they could be Vladimir Horowitz but if you hate them you'll never learn.
-A good long term goal is key. But wait on it, if you try to go for Chopin's Ballade 1 (as my friend once tried to, as did another member of this forum) with no experience, you will likely damage your fingers and your musicality, and for what? It will 99.9% of the time be god awful. A good way to work your way up is select your dream piece, and then figure out what its technical and musical challenges are, then figure out some pieces to help with that (with the help of your teacher of course; and in general, you may need to wait a bit before you do that, and just focus entirely on what your teacher selects for you). For example, my piece for a long time was Moonlight Sonata movement 3, so I played the first two movements to get a feel for the form, then analyzed the sonata and saw the repetitious movements, so I did a lot of arpeggios and things like that. Now I'm just building my overall technique.
-17 is a fine age to start. I know somebody who started at age 50 and is a fine pianist, in fact I'm going to a show of his where he's playing the Appassionata sonata. However, it may be harder depending on how you look at your learning. You'll see the 14 year old kids playing the Pathetique sonata or Hungarian Rhapsody 2, and it may motivate you to practice, or it may make you think "jesus, I'll never get there!". A little bit of inspiration is good now and then though, just don't get so caught up in the future that you forget what you're doing to get to that goal. Chopinlover01 can tell you about that