If you're just starting out, the first decision you need to make is what sort of bike you want to start with.
The world of cycling is a big one and it caters for all tastes - from skinny wheeled speed racers to heavy duty all terrain bikes to unicycles! And you can even get bikes that you can fold up and stick in drawer!
It is totally up to you and most bikes shops will stock the full range.
Things you need to take into account are:
1. how much you want to spend
2. what you will be using the bike for (distances, terrain, frequency, social/travel/competition)
3. how you will store the bike
A general guide
Town bikes
Road bikes
"Hybrid" bikes
MTB (mountain bikes)
BMX
Track
Recumbents