I like Charles David's basic pumps (round toed for me, since I have wide feet), and I'm a pretty preppy person. I think a black pair and a tan pair of pumps are key. I'd stay away from Mary Janes for now, unless they're the Manolo pointy-toed ones- otherwise, I think they'd make you look too young.
I'd hold off on the boots. I live in LA, and I rarely wear mine. Instead, I'd get a pair of black, brown, or tan *flats* for work (depending on what would be most compatible with your wardrobe).
In terms of casual shoes, I wear my Reef flip-flops all the time. Work shoes hurt my feet enough; I'm not going to feel a lot of pain on the weekends. Those run only about $20-$30, so that will save you a ton of money for more work shoes.
As for evening shoes: personally, I tend not to invest a lot in these, because I don't trust my drunken self to not ruin a pair of nice shoes. Maybe go to DSW or some other shoe outlet and get a pair of Beverly Feldmans or Ninas.
Beyond those pairs of shoes, I'd suggest you browse the local sales and shoe outlets, and grab good deals on basic pumps and flats. I'd also consider getting a pair of Aerosoles, Born, or Easy Spirit shoes (do NOT laugh at the last one: Easy Spirits are NOT only for old women- I'm in my 20s and will happily wear their more stylish shoes- your feet will thank you). For me, adjusting to wearing "work" shoes (pumps) all the time was difficult, and my feet would get so blistered (and, at times, bloody). I think you should invest in a comfortable pair of work shoes, because you may have that problem too.
Good luck!
ETA:
- DON'T buy Nine West shoes. They wear VERY easily. I have a couple of pairs and really have to baby them. Better to spend a little more on, say, Charles by Charles Davids or Stuart Weitzmans, as they will last much longer. All about cost per wear.
- DO look at
amazon.com on a semi-regular basis. They have the best. sales. ever. - especially if you know your size in a particular brand. I purchased 15 pairs of Charles Davids there last spring for $30-$35 each.