Congrats hun on the birth of your beautiful baby girl!! And its nice that you are waiting til shes a bit older to return to work.
Are u planning to place her in childcare and how many hrs will u b working when u return to the workforce? Both ways you stil breastfeed (if u prefer), just use a breast pump to express milk into her bottles then refrigerate for up to 12hrs (either this or 24hrs, advisable to check with your health professional) and if you want to keep the breast milk more fresh or you are working long hours, it can be freeze and heat up when needed.
Cows milk is another alternative and can be introduce at 12months. Good luck and take care.