First thing is to make a list of what you want. If it's a basement renovation then write down what its uses will be such as work out room, entertainment room, etc. If it's a bathroom you can have all the standard things like toilet, sink and bathtub but you can specify things like tile shower, granite vanity top and so on.
Second thing is to make a drawing or two. It can be hand drawn with basic measurements just to get the point across. This helps when you are getting estimates from contractors so they can visualize what you'd like.
Third is to talk to a few renovation companies to get some prices. Make sure that you have given everyone the same information about the work you'd like done. Ask them lots of questions and get some information about their company. Ask your friends to recommend contractors they have worked with in the past.
Once you have some prices make sure you compare their scopes of work and not just the price. Sometimes a low price is an indication that they have missed something or that they may not provide the quality you are looking for.