Understand what kind of bike you need. This depends on the type of riding you are going to do. Are you riding around town then get a city bike or hybrid, or are you riding in the forest and trails then get a mountain bike? Do you prefer to ride fast or just comfortable? If you prefer riding fast then get a racing bike.
Make sure it fits
Fit is important to be comfortable and enjoy riding.
Get the right size. The controls must be easy to reach. You can make adjustments yourself but keep in mind that the frame size is fixed. So get the right frame size before you buy it.
Understand the different Bike Types
There are mountain bikes, city bikes and hybrids and many brands. Don't get fixed on one brand only.
You want a light Weight Bike?
Then you have to pay more
Do you want an Electric Bicycle?
Make sure you get a decent battery that can get recharged often.
And keep an eye on the weight of the bike and your bodyweight. If you are too heavy get a higher voltage battery that goes along with a stronger motor.