Setting a fan baseplate is not too difficult. Before you install the new base plate you want to take a look at the wiring in the box. Many newer fans will have an option for having the light and fan on separate switches.
Did your old fan have this option? No, then it is likely you don't have enough wires to make it work. You need an extra switch wire in the box.
You should get a new base plate and screws with the new ceiling fan. There is something else in the box that you will need to take a look at. Yes, it is the instructions. The mounting of the base plate should be fairly straight forward, but it is good idea to check.
The new fan will have a sequence that the parts need to be installed in. Not following the instructions can result in taking it back apart. Not fun.
When you are wiring a ceiling fan, support for the electrical box is important. The box needs to be nailed or screwed into a framing member. Ideally, extra wood blocking should be added to support the fan and heavy duty screws should be used to attach the box to the blocking. A box that is just attached to drywall may support a normal fixture. It will not support a ceiling fan.
You should be all right if you had a ceiling fan there before. Changing from a standard fixture to a ceiling fan could be a problem. Check the weight on the new fan and see if you think the box will support it. You could wire a pail of sand or something else heavy and do a test. Just a thought.
Assembling the Ceiling Fan
Assembling a ceiling fan will be a lot easier if you follow the instructions. There are generally four basic steps involved:
(1) Installing the motor unit. (2) Wiring the motor and light. (3) Installing the fan blades and (4) Assembling and installing the light.
At this point you will need to look at the instructions. Every fan is different and different manufacturers have different ways of doing things.
The sequence of installing certain parts can be critical. As a rule you put together the motor unit first, wire it and then attach the blades. The lighting will usually be last.
At this point you want to assemble the motor unit with the trim rings. After you get it put together you will hang it on the maintenance hook. Hope you got one. Otherwise you will need someone to help you while you hook up the wires.
The box won't support the weight? Not good news. You could cut a hole in the drywall and add some blocking between the joists or rafters. This will leave you with a drywall repair to do. See the article 'Drywall Hole Repair', for instructions on how to repair the drywall.