Welcome to THR, retiltxet! Very nice choice for a first handgun! My first was a Sig Sauer P220, so we share the same fortune for being able to get a quality pistol as our very first; don't get rid of it because it will hold sentimental value in the long run.
As for a spare magazine, it is almost a necessity if you are going to carry concealed. It's not because you're going to get into a drawn out gun battle where you'll be exchanging magazines and shooting to slide lock like Mel Gibson (but who knows in this day and age, right?), but also in case you experience a malfunction with the magazine that's in your pistol. What if you accidentally hit the mag release button, dropping that precious loaded magazine to the floor and cannot retrieve it? What if the magazine, for whatever reason, fails to feed or breaks at the most inopportune time? The spare magazine tends to balance out the load a bit more, too. Once you get used to it, you'll feel naked without it.
When it comes to holsters, it's all about personal preference. There are many fine holster makers out there who do quality rigs, so the choice is yours. Be aware if you choose a true custom maker, there may be an appreciable wait time for your rig to be built, but it's worth it in the end. I recommend you buy the best holster you can afford, and put the extra money into an actual gun belt - it will help greatly with carrying the pistol evenly and comfortably. The standard belt you'd wear with your jeans tends to be not strong enough and can make or break what you find comfortable with carrying a pistol.
Ammunition is also personal preference. If you do a search for defensive ammo, you'll get multiple opinions over what people prefer and use. Bottom line - whatever you carry, make SURE it functions 100% in your pistol. Run as much defensive ammo through your gun as you can afford and what you deem necessary. I don't qualify a full magazine as being 100% reliable, but it can be costly if you choose to run 100 rounds through it for peace of mind. Top contenders right now tend to the the Winchester Ranger and Federal HST line. Both are excellent performers and available via numerous online sellers.
Congrats on getting your permit, and for your very fine first gun! Be vigilant and be safe!
My Sig P229, slide done by Tripp Research:
