i want to know if i were to fix a kitchen cabinet, what are the basics that i need to know? currently, saving money to fix an extra kitchen cabinet (above the existing sink area and kitchen floor cabinet); have no idea about it....so need

i might want to fix a kitchen cabinet; but how do i go about? do i go to those cabinet shops to look for those cabinet design that suit my kitchen and they will have contractor to fix it for me? Or, do I look for a contractor first and then he recommend the designs? which way better?