The front derailleur is a key components to ensure smooth shifting between your front chainrings. These can be made up of two or three rings- known as a double or triple chainset. So with this in mind make sure the front mech is compatible with your chainrings.
The next thing to consider - is your frame braze on compatible or band on? Most modern new frames are braze on compatible whilst older bikes, steel bikes and still many modern bikes are band on. Braze on means the frame has a built in facility on the frame to accommodate a front mech, this would be directly screwed into the frame. Whilst a band on means the front derailleur is attached onto the frame using a clamp. Make sure you know which your bike can take before you choose.
Finally, which make and model do you choose - This all depends on what groupset you are running. If you are using Sram then it is best to use Sram components throughout. The risk of mixing brands up means you may not get the best possible shifting. Some users may upgrade some of the parts to help with weight saving and seamless shifting. For instance, you may have a Shimano 105 groupset and upgrade to an Ultegra front mech.