Scoreboard Things

/Scoreboard cheat sheet

OBJECTIVES

/scoreboard objectives add <name> <type>

Most used Objective types:
Not updating: dummy
Modifyable: deathCount, playerKillCount, totalKillCount
Always Matching: health, food, level, armor, xp

/scoreboard objectives setdisplay <display> <Objective>

Most used displays: sidebar, belowName, list

/scoreboard objectives list

This will give a list of all created objectives

PLAYERS

/scoreboard players set <Player/Selector> <Objective> <Score>
This sets the score to specific amount

/scoreboard players add <Player/Selector> <Objective> <Score>
/scoreboard players remove <Player/Selector> <Objective> <Score>
This adds or removes from current score

/scoreboard players reset <Player/Selector> <Objective>
This will remove the score

SELECTORS
Replace "Objective" with whatever objective you want to use

@a[score_Objective_min=3]
This will select all players with AT LEAST 3 (or whatever you fill in)

@a[score_Objective=3]
This will select all players with AT MOST 3 (or whatever you fill in)

@a[score_Objective_min=3,score_Objective=3]
This will select all players with EXACTLY 3 (or whatever you fill in)