When you first install and load Lightroom CC for the first time, it should find your existing catalog, convert it and you are ready to go in LIghtroom CC. You shouldn't have to do anything else to have your photos available in Lightroom CC
Edit: It does ask you if you want to upgrade the existing catalog, you should click Yes or OK or whatever it is.
You may want to look at my post here:
The above procedure allows you to retain your previous LR5 standalone version until you have tested the CC plan version. I do this with all major Adobe app upgrades until I am sure all bugs have been identified and fixed. Unfortunately with LR 6/CC you need to do a little extra work to prevent losing your previous version Preview files. Make sure you have sufficient hard drive space to hold the duplicate Previews folder, which can be quite large.