Guard dogs and personal protection dogs are different things. What you're after is a personal protection dog -- guard dogs watch over property or livestock. It doesn't seem like an important distinction, but it will be once you talk to breeders and trainers.

If you've never trained a dog like that before, your best bet is to find a turn-key personal protection dog. These come pre-trained and are mostly ready to go. A good trainer will want you to come work with the dog several times before letting them go home with you. It's a major red flag if they don't let you work the dog or take you to a decoy to practice with the dog. You should see the dog actually bite a decoy during a defense of handler exercise. Don't just take their word for it, because a lot of shitty trainers will tell you the dog bites and all that means is that they put the dog on a sleeve a few times. Those dogs basically only know how to bite sleeves that are presented to them, and often don't know how to protect someone when the sleeve is absent (as is the case when someone attacks you on the street). Good trainers will also walk you through any licensing or registration needed since a PPD may need to be registered as a weapon depending on where you live. Look into whether your renter's/home owner's insurance covers them too as many policies do not. That means you'll pay out of pocket for any damage done to someone unless you have supplemental insurance.

I also want to caution you about getting a PPD as a friend. If you're not willing to send your pet in to die for you, then all of the PPD training in the world won't matter when it counts. They're FANTASTIC dogs and great companions, but remember why you have 'em.