If you are just getting into the business I suggest that you invest in some of the CDG disc sets available. These include the Chartbusters Essential 450 Pack, The DKK Millennium Set, and the Sound Choice Foundation Series. All of these contain a good selection of the more requested songs. If I could afford it I would buy every Top Hits Monthly discs released in the last 12 months. This would give me a great collection of the latest hit songs. In fact, I would say that my Top Hits discs get used more than any other brand I own.
There are many karaoke CDG brands available of varying quality. The ones I mentioned above are among the best. You should keep an eye out for deals from clearances and discontinued brands. This is a good way to increase your song collection at a low cost. That doesn't mean that you should purchase songs just because you don't already have them or because they're cheap. There is no sense in loading your song books with songs that nobody will ever request.