Sounds as if your clutch isn't releasing. If you're certain the cable is tight you can try this old method for freeing a new clutch before going any further.
Remove the clutch actuator cover, (left side of engine where the chain enters the sprocket). In the center of the sprocket is a steel pin which protrudes about 1/4" Try hitting that pin with a hammer. Don't hit it like you're driving a nail but give it a good rap. Sometimes the clutch pads are stuck from the factory and this shock against them will free them up and allow the clutch to disengage. Get back to us if it doesn't work. There are other things to try. By the way, that steel pin, and a ball bearing that is behind it will come out fairly easily. Don't loose them. Glob some grease on those partrs as well as the cam in the cover while you have the cover off.
As for chain tension, you want about 3/4 to 1" of slack measured on the top portion of the chain with the bike being pushed forward. Just an afterthought, make sure your chain isn't bunching up in the clutch actuator cover/sprocket area. Loose chains can do that and cause binding of the rear wheel just like an engaged clutch.
Tom