DO:
DON'T:
Uh, that's it from me. 5e is "balanced enough" that there's not really much more to it than that.
- Identify your primary attack stat and try to get a 16 in it via racial mods. Usually it's obvious: Str or Dex for warriors, spellcasting ability for spellcasters. Increase it to 18 at level 4 and 20 at level 5.
- Perception is far and away the most rolled skill, and is sometimes very, very important. Consider selecting it as a proficiency. Or not; some people are very happy playing an oblivious character. But the party should probably have a few members with decent Perception, unless you really like stumbling into ambushes, missing clues, etc.
DON'T:
- Don't use Constitution as your dump stat. If you want to be a warrior-type, you'll want even more Con, at least a 12+. Hit points matter, and you will never have enough.
- Don't choose obviously suboptimal equipment or spells for "role-playing reasons." A pacifist wizard or fighter who insists on only using a quarterstaff... when an RPG has a strong class system, playing against type is complicated.
- Don't even consider feats or multi-classing until you've got some game-play experience under your belt. These can get complicated and can gimp your character in certain ways if you are not careful.
Uh, that's it from me. 5e is "balanced enough" that there's not really much more to it than that.