You bought a good, budget, reliable starter AR15. If you never get black rifle disease, that gun will serve you fine for the rest of your life. Should you get the disease, I would recommend a mid-length gas rifle, probably with a free float rail, for your next gun. If you plan on using it for home defense, then it needs a flashlight mounted. I can recommend the Elzetta ZFH1500 mount, if on a budget. The Impact Weapons Components flashlight mounts are a bit more ergonomic, and a bit more pricey, but I can recommend 100% as well.
1)For mags, the best GI aluminum mags are made by NHMTG/Okay. They discontinued the NHMTG name and are now sold as Okay brand. The fit and finish is better than any other GI mag on the market. D&H mags are serviceable and reliable, but they have sharper edges and are not as nicely made. Brownell's mags are nice as well. Make sure they have Magpul anti tilt followers (if 30 rd or curved 20 rd mags). If its a straight 20 rd GI mag, then you cannot install Magpul anti tilt followers as the follower is designed to tilt on the 20 rd straight mag. That being said, the 20 rd straight GI mags are excellent and super reliable. One of my favorite mags.
Magpul should be considered top tier mags. They fixed a lot of the short comings with the GI mags, including modifying the feed angle to be more reliable than GI/other mags. The 2nd gen MOE and 3rd gen mags will work 100%. If I was new to the game like you, i would just buy Gen M3 PMAGS. They are affordable, durable, and reliable as an anvil.
Lancer mags are durable and reliable, with a small problem. If you pull the charging handle back slowly while loading, they will jam up. GI mags and Magpul mags dont have this problem. Lancer followers are also much more likely to get sticky in the magazine if sand or dust is introduced.
Basically, as far as mags go, just get Magpul mags, and a few Okay mags so that you can have some good quality GI aluminum mags.
2) There is no special procedure needed to put the gun on safe. All you have to do is make sure the hammer is cocked. Rack the charging handle (to cock the hammer) and flip it to safe. Done.
2a) The gun has a floating firing pin. It strikes the primer of the round every time a round is chambered. It will even leave a very small dent in the primer. Don't chamber the same round more than twice.
3) The plastic parts are durable. The plastic rear sight by Magpul has been tested to be MORE durable in drops than most of the aluminum backup sights on the market. Its cheap, but it works, and its durable.
4) You'll never every use the forward assist. I've put north of 50K rounds of .223 down range and I can never remember an instance where I used the forward assist. If the round wont go in, eject it, and start over. The Army insisted it be put on. The Airforce didn't want it. Eugene Stoner, the designer of the gun, hated the forward assist and thought it was unnecessary. The Army won that argument.
5) The guns are not delicate. Contrary to popular belief, they will run pretty well when dirty as long as they are well lubricated. Its better to over lubricate these guns than under lubricate. It will throw any excess oil out of the action if there is too much oil. Its almost impossible to over lubricate them. These guns also prefer a heavier viscosity lubricant, over a thin lube, due to the heat of the hot gasses blowing back into the action. Oils that I have used, and can recommend for this platform, are Weaponshield, Breakfree LP (not the thinner CLP), MPro 7, Slip 2000 EWL, Lucas Extreme Duty gun oil.
6) The most important thing to clean on the AR is the chamber. Many folks just run a .22 caliber bore brush down the barrel, which cleans the barrel just fine, but completely misses the chamber. An AR15 chamber brush is an absolute necessity, in my opinion. If you buy no other cleaning accessories, make sure to get an AR15 chamber brush.
7) AR's need proper lubrication. The below pictures are the best I have seen about where to lube. These are the recommended lube points straight out of the govt manual for the gun.