Extraction of premolars generally gives the most amount of space for correction of crowding. Whether this is needed will depend on how much space is needed. Space is required for straightening the teeth, leveling the bite, midline correction and to reduce protrusion of the incisors.
The choice whether to extract first or 2nd premolars depends on how much anchorage (resistance to unwanted tooth movement) the case requires and which tooth is compromised (large fillings, root canal-treated, crowns, cracks/fractures, impacted or in a very poor position).
Generally speaking, removing first premolars gives greater anchorage that can allow greater incisor retraction that may be desirable in certain cases. That being said, anchorage can be supplemented with other methods (like mini screws/TADs).
Losing anchorage may sometimes be a good thing, depending on the case.
Ultimately, this is something that the treating dentist needs to have an idea of before starting the case and through analysis of your case's requirements.
