I have the opportunity to restore a 1965 Mustang Fastback. My dad drove it into our basement and parked it. And there it has sat for 20 plus years under a cover. Only thing that may have happened is if mice chewed some wires but I haven't taken a good look at it yet. Where do I begin? Drain the gas? Drain the oil? Just a tune up and go from there to see what else it may need. Or do I need to take more precautions before trying to start?
A couple of places to begin: 
Before you begin it would be a good idea to give the car a good going over to determine whether it's a candidate for a complete restoration, or to decide if it just needs some sprucing up. I would certainly start with new gas and oil. You might also want to flush out the cooling system. 
If you think it's driveable, first you would need to replace the tires. If the car has been sitting for over 4 or 5 years, there's a good chance that the rubber is dry-rotted. Also, I would pull the wheels off and look at the brakes. Even if the brake shoes/pads look okay, I'd replace the brake fluid and check the system for leaks and make sure it functions correctly. Keep in mind that there was a reason that the car was parked and left for 20 years. It may not have been road-worthy when parked. You may want to find an online (or local) Mustang club. There are a lot of people out there that are willing to offer advice and help when they can. There's nothing like experience and a lot of folks have it and are willing to share it.

If you decide that the car warrants a a complete restoration job here are some steps to consider:
1. Get a copy of the shop manual for your car.
2. Take many, many photos of the car before you remove the first bolt. I can't emphasize this enough. It'll be invaluable when it comes time to put it all back together.
3. Honestly assess your technical skills, and with that assessment in mind, give the car a good going over (when taking photos) to see what work required you can do, vs what you'll have to have done. There will most certainly be some parts of the car that will require refurbishing that you won't discover until you begin taking things apart, so this appraisal is primarily to make sure this is a project you're comfortable doing. Cars are easy to take apart....they're much harder to repair and put back together. In your initial assessment, you also need to decide if you have the space to work on the car and if you have the money that may be necessary to bring the car back to where you want it. Car restoration is an expensive hobby. On the plus side, a Mustang is a good beginners project, because parts and service are readily available for this car. I'm not trying to dissuade you from taking on this project, but you have a good picture of what will be required.
4. You will want to keep a log of everything you do to the car. This can be created on your computer, but be sure to print out a hard copy and keep it in a notebook. This helps you keep track of what your total investment in the project is. If you  have a scanner, you should scan in all the receipts for the parts you purchase and add them to your notebook. You should keep the originals, in a folder, also. You can also use it to track what parts are good, need to be repaired, or have to be replaced. And you can keep track of where you buy your parts. Besides giving you a good accounting of your project, it also creates a good presentation piece should you try to sell your car and you can offer it as confirmation that the work has been done.

Be methodical, and organized and your project will go easier and be more rewarding than if you spend all your time trying to figure out how things go back together, or looking for parts that are scattered around your garage.

Good luck with your project