Avoid neckbeard unless you’re going full on lumberjack. Tilt your heard back, put your index and ring finger just above your Adam’s apple. Follow the curve back towards your ear and that should be your shave line. Then put your chin to your chest and you’ll see where it needs to be shaped/blended. Alternatively, you can go to a barber for a trim and pay attention to what they do and copy it.
I use the same shampoo as my head, whether it’s head and shoulders or some fruity stuff of my wife’s or maybe even the dog’s. Okay, not the dogs, but I have used the kids stuff. I’m a bar of soap guy; there’s no “regimen” of scrubs and moisturizers and whatever.
Depending on how much oil you naturally put out, you may or may not want oil or balm. It definitely help reduce the itch, but use sparingly. I use balm, but that’s just to try to tame how my hair grows on my right side...a hurricane pattern. If your balm gets dry/cracked, boil some water and drop the jar in for 5-10 minutes. Take it out, shake a little and then into the freezer. It’s mostly beeswax, so it’ll melt and resolidify when cool.
You’ll find what length works for you based on 1) how it looks and 2) when it stops itching. I can’t do a stubble beard, it’s too itchy and shows my tornado pattern. I can’t do a medium-long beard, it doesn’t come in quite full enough to not look scraggly. So my zone is 12mm-16mm. Know that sometimes a beard needs to get through an ugly stage to grow out to a better look.
Finally, how you dress will alter how your beard looks. Take Keanu Reeves. He has a terrible beard. Spotty and patchy. But it looks okay-ish because he normally dresses well. It does help he’s famous and people recognize him, so they automatically know he’s not homeless.
EDIT: Damn, just saw this was a necro thread. I’m so ashamed. I’m usually so good about paying attention to date stamps.