It sort of depends on what you want to observe and how picky you are. The
problem is it is hard to answer those questions until you have done some
looking through telescopes. I'd recommend going to:
http://skyandtelescope.com/resources/organizations/ and finding your local
astro club. Go to their public nights. You'll get to look through a lot of
different telescopes and see for yourself what you like. And, there is
frequently a good starter scope around to buy used from someone who is
upgrading. Many clubs also have loaner scopes you can use for a month or
two. That will give you experience as well as show you what you personally
are going to want to insist on in a scope and mount.
Welcome to a great hobby!
This is the answer that I think is most useful to a newcomer. The simple
answer is you only have to spend about $20 to get into astronomy (or
whatever the annual subscription is to your local astrosoc for talks,
events, meeting etc.). It is especially relevant to teenagers interested
in science and we desparately need more of them. Astrosoc populations
are all aging.
Most societies have experienced observers who will be happy to show you
the skies *and* many have small loaner scopes begging for someone to use
them. It doesn't make a lot of sense to buy your own scope until you
know what you want to do with it. They make expensive doorstops...
And for a first scope you can do pretty well secondhand if someone who
knows what they are looking at goes with you to examine it. Astronomy is
one of those hobbies where plenty of people invest heavily and then get
bored or give up after not very long.