It depends if you have the supplies or ability to steep specialty grains and also depends on what type of stout you want to brew. Judging by your want to make a chocolate stout, I'm going to assume you want it sweet. Therefore, I would pass on black patent malts, roasted malts etc. You want to keep your bittering agents to a minimum.
What I would recommend if you CAN steep, is to get some Crystal 60-120 malt (also called caramel malt), some chocolate malt (Not necessarily cocoa flavored), maybe some biscuit or toasted malt and steep from room temp to about 160 degrees. If you can, let it steep for 30-45 mins around 160 deg then pull and trash. Then begin your normal boil from there. As far as the proper extract, I would use a light or maybe amber extract if you steep.
If you CANT steep: I would use the dark extract. That's really all you can do in this instance.
Keep your hops low. I would recommend 20-25 IBU MAX. Also, if you want sweet chocolate flavor, and not bitter cocoa flavor, opt for chocolate extract and NOT cocoa powder. Chocolate extract is BEST used in secondary or at bottling. Furthermore, to achieve more sweetness, I would add lactose. 1lb per 5gallons. It adds a thickness to the mouthfeel and adds lasting sweetness as the sugars are largely unfermentable.
Hope I helped.