I also suggest you check
CSM rumor threads as I'm planning on starting them myself and am doing the same.
One thing you can do is try and come up with a play style. Do you like to run up and chop things to bits? Do you like to hang back and shoot? Find your play style and choose Gods that match it. Feel free to mix and match as well. ex:
you like to chop things up, so you have a couple of squads with khorne icons in rhinos to dash up and chop things to bits. However, so you can actually win games (Who doesn;t want to win?) You may include a Nurgle squad just for holding objectives and giving support fire.
another thing to decide is if you want a god-themed army (world eaters, death guard, emperors children, thousand sons) or a chaos undivided (Black Legion, Iron Warriors, Nightlords, Alpha Legion). As Cult Marines will be elites, you need to decide this before hand so you can invest in the proper lord to unlock them as troops. Also realize you cannot mix cult units as troop choices, only one cult unit is considered a troop unit per game, if that.
Finally, as the new book i about a month away, If you get the hobby jitters, buy something thats rumored to be good. Predators will be nice with the new shooting boost, Oblits too. And one can never go wrong with a rhino, 3 hull points actually may make it more survivable. If you do get a rhino, pick which squad will use it and paint their god symbol on the hatch (or somewhere else easibly visible) as this will help in the long haul (rumored marks for vehicles, transport gets same as squad)
Good Luck