One of the first things you will need to do is to take off your old cabinets, when taking them off the wall remember that there is often not only nails holding down your cabinets. There is very often glue or some other substance holding the wood to the wall, dont take the wall off with your cabinet. Ordering a cabinet means that everything will usually the draws will come in the cabinet already, remove these befoer you begin to install the cabinet, this will make life alot easier for you. Installing a cabinet is usually a two person job, but for some extra help buy a slate of wood and screw it into the wall to support the weight of the cabinet when it is on the wall.
When you are aligning your cabinets make sure that you mark at the corner, this will ensure that everything is at the correctvangle and height. Remember to also buy shi,s for your cabinets, these are very important and are someitmes forgotten. These shims are used to keep the cabinet straight and that there is no space for shifting around when you are screwing in the last screws.