Members of this forum are luckier than newcomers doing it alone as we have a pool of some experienced observers here to ask for advice.
When I started out a few years back I did many hours of research on the internet reading a lot reviews and various yahoo forums and consequently haven't made too many expensive mistakes.
The ones I have made involve budget wide field eyepieces which simply dont cut it for me in an f5 scope. One I just sold for $45 originally cost me $140
Another eyepiece I just sold is a televue panoptic, which I just sold for exactly the same price I paid for it over a year ago.
The lesson is that quality gear holds its value and you almost always loose your money when you buy trash.