It depends how 'serious' you are about changing that room.
In which case, if you really want to do the real deal!...Then you need a blank-canvas.
It also depends on the room e.g. Lounge room, or bedroom.So say if it was a lounge room,you move out all the clutter, and move furniture over one side.Then pick a light colour, as you may find anything dark...will and can make the room smaller......Two coats of paint is better.Never use a cheep roller, the paint just gets trapped, in the roller, only to drip every where as it becomes heavy.It makes the walls look really bad.Once you have painted your room, then you may like to try and keep it to 2-3 main colours.That means, your lounge suite one colour your blinds or curtains same as the walls, and a rug that colour too.
Too many colours in a room, is not sleek...but messy.Have a look on Google Images - just type in ''lounge room colour ideas''.It will give you a great choice of colours, and show how by using just 2-3 colours makes a room.....Have fun! :)
If the paint needs updating, then this is a great place to start as grubby walls can really bring down a room. Next, see what you can do with the furniture you already have, sometimes just rearranging things and getting rid of clutter can be enough to make a space look better.