Hard to give you a basic army list, perhaps letting us know what units you like the look of, what models you have/plan on getting and what play style your looking for.
To help out I'll go over the troop choices we have available, a very health five different options
as these will define your army.
Strike squad - A good solid marine unit, generally need a transport and even though they have power weapons they are a shooty unit not a close combat one.
Terminator squad - More expensive than the strikes but more survivable and better in CC, don't need a transport as they can DS and are more protected when walking.
Paladins - Unlocked as troop by taking Draigo. These are the ones to go for if you want a super elite army, very expensive but can take multipul heavy weapons and are extreamly good in assault. Makes for a very easy on the wallet army but is vulnerable to certain other army builds.
Purifiers - Unlocked as troop by taking Crowe. These guys are brilliant, great at shooting with lots of heavy weapons, great at killing both elites and hordes in combat. Generally each unit will need a transport to get around in as they are expensive and just as easy to kill as a normal marine. A very good all round choice but you do have to take a not very good character to use them.
Inquisitorial henchmen - Unlocked as troops by taking Coteaz. These guys are super versatile and very cheap. You can load them out to be good long range shooty units or assaulty units of death, all are reasonable easy to kill so makes for a very different type of GK army. Best to have a good read through there army list entry and get a feel for them as they have so many options.
Hope thats of some help.
