Some quick tips to help you get started fairly easily with stuff you will use. For the Circle player I would suggest picking up
Shifting Stones, plastic heavy kit, druids of orboros, gorax.
The shifting stones and gorax will be in almost any list you make in Circle, while the druids give you a decent unit, and the heavy kit allows you more options for a heavy beast.
Heavy kit, shepard, forsaken, striders
Again the heavy kit offers you more options (if you are not magnetizing I suggest making it into a ravagore), the shepard brings one of the best fury management options in the game. The forsaken is another good solo and will see use in a large number of early lists, lastly the striders are a good unit that gives you some options in lots of games. Hope this advice helps.
Please Help Level My Daemons. See which one will level faster, will Papa Nurgle get his way, or will the Prince of Pleasure rule?