Glasgow Airport is very easy to get out of. Once you pick up your car, just follow the signs for the M8. It's in a roundabout, and the ramp out of the roundabout comes up on you really fast, so you may end up going around the roundabout twice. Once you get on the M8, it will take you toward Glasgow. The airport is actually eight miles outside the city, so you have the option of heading in to or through the city. Unless you arrive during rush hour, you shouldn't encounter much more traffic than you would coming out of Edinburgh Airport.
If you want to head toward Edinburgh, just stay on the M8 - it goes straight there. Or, if you want to head directly to the Highlands, take the M8 west toward Greenock, cross the Erskine Bridge, then take the A82 toward Dumbarton and Loch Lomond. The A82 will take you wherever you want to go in the Highlands. If you end up on the M8 east, and want to go to Stirling, look for signs for the M80. That'll take you there. If you're heading south, follow the M8 east until you get to the M74. It'll be signposted Carlisle.
Stay in the left and center lanes. All of the exits are from the left.