Firstly, you should buy a pushchair or pram that lies flat. This is important to prevent damage to your baby's back. If your baby suffers from colic you might find s/he is more comfortable at a slight angle, so look out for a pushchair that will allow you to adjust the position to different settings.
After considering baby's comfort you should decided how you plan to use the pushchair in your day to day life. Will it need to be folded and stored in your car, will you be walking long distances, will you be shopping with baby?
Consider buying a pushchair you can easily lift, fold and unfold with one hand whilst carrying your baby in the other. Practise in the store before you buy.
If you are planning on doing a lot of walking then sturdy wheels with thick rubber "tyres" are essential. I've had several wheels wear out and give up on me. Bear in mind that cheap lightweight strollers with small wheels wear out much faster than big pushchairs with big wheels, if you are on a tight budget it is probably cheaper in the longrun to splash out on a sturdy one to begin with.
If you plan on walking over rough terrain eg cobbles, grass, etc, consider a pushchair with suspension and lockable wheels. Swivel wheels are great on flat ground but almost impossible on grass.
If you plan do to lots of shopping with baby then look for a big shopping basket, and a storage tray on the handles is very useful for holding your keys and change.
If you plan to travel by car and then walk, a travel system is a brilliant choice as you can transfer the car seat to the pushchair without disturbing your sleeping baby. Even if you don't have a car you could find the seat useful, although car seats don't lie flat and its not recommended to use them for prolonged periods for newborns.
Finally, if you still can't decide, remember that you have to push this pushchair around with you for a long time, maybe two or three years. Pick one that you like the look of, that won't go out of style to quickly. It might be sensible to choose a neutral colour because although baby girl might look gorgeous in a pink pushchair, you may want to reuse it for your next child and a little boy might not be so well suited.