How to send message to the player ?

The all commands to send messages:

Target selector: @p | @r | @a | @e | @s | playerName

Raw code

/say Hello world !
/tellraw @p {"text":"Hello world !"}
/title @p {"text":"Hello world !"}

Example with /say | /me | /tm:

/say Hello world
/execute at @p run say Hello world !

Example with title | teelraw

/title @a title ["",
  {
    "text":"Hello",
    "color":"aqua"
  },
  {
    "text":" world",
    "color":"dark_red"
  }
]
/tellraw @a ["",
  {
    "text":"Hello ",
    "bold":true,
    "color":"dark_red"},
  {
    "text":"world"
  }
]

Advanced

/tellraw @a {
    "text":"Hello world",
    "bold":true,
    "italic":true,
    "strikethrough":true,
    "underlined":true,
    "obfuscated":true,
    "clickEvent":{
      "action":"run_command",
      "value":"/say hello"
    },
    "hoverEvent":{
      "action":"show_text",
      "contents":"Hello world"
    }
}