Agree you should go into your local tackle shop and show them the rod and the reel you will be using. If you are new to spey fishing they can help get your rod and reel set up properly. You will need backing and if it is a full spey line, then attach it to the backing. If it is a shooting head, then you will need backing, running line, and then the spey shooting head. As far as the grain weight to use, if your rod says 650 grains that is a good place to start. If you are using a skagit line, just go with a 650 grain skagit spey line. The 12 to 15 foot tips you will use on your rod do not count in the line weight as you will be using a waterborne anchor to cast the line. If it sounds confusing ask the people at the fly shop to go over it with you.
If you are new to spey casting, you may want to take some lessons, or look at some of the instructional videos on you tube. Have fun.