Basically you can get the Player by the Character, example when a player touches a part, then GetPlayerFromCharacter() can be used, but now i need to do the opposite, and get the character from player.
Can someone help me please? 
Once you have a reference to the Player object/Instance, Player.Character can be used to reference their Character Model.
As an extra precaution, you may want to define their Character as
local Character = player.Character or player.CharacterAdded:Wait()
in case there’s a circumstance where you may need to wait for the player’s Character to respawn/load before performing further actions in the script.
Edit (November 1st, 2021): Here are some examples that might be useful!
local Players = game:GetService("Players")
local player = Players.LocalPlayer
local Character = player.Character or player.CharacterAdded:Wait()
Additional Resource: CharacterAdded Event - Creator Documentation
local Players = game:GetService("Players")
local player = Players.LocalPlayer
local function updateHealthBar()
-- Example function
end
local function characterRespawned(Character)
updateHealthBar()
local Humanoid = Character:WaitForChild("Humanoid")
Humanoid.HealthChanged:Connect(updateHealthBar)
end
player.CharacterAdded:Connect(characterRespawned)
if player.Character then
characterRespawned(player.Character)
end
Additional Resource: PlayerAdded Event - Creator Documentation
local Players = game:GetService("Players")
Players.PlayerAdded:Connect(function(player)
player.CharacterAdded:Connect(function(Character) -- The parameter of the function will refer to the player's Character in this case because the activation of the CharacterAdded event sends the Character instance to the function
print(Character.Name)
end)
end)
Additional Resource: TriggerEnded Event - Creator Documentation
local Part = workspace.Part
local ProximityPrompt = Part.ProximityPrompt
ProximityPrompt.TriggerEnded:Connect(function(player) -- When the ProximityPrompt has been activated, the TriggerEnded event tells the function which player activated it
local Character = player.Character or player.CharacterAdded:Wait()
end)
Additional Resources:
Team.PlayerAdded Documentation - Creator Documentation
Loops and Arrays Guide - Creator Documentation
Players:GetPlayers() Documentation - Creator Documentation
local Players = game:GetService("Players")
local Teams = game:GetService("Teams")
local function randomizePositions()
for _, player in ipairs(Players:GetPlayers()) do -- Loops through the Players Service which contains every player Instance in the game
local randomXCoordinate = math.random(-100, 100)
local randomYCoordinate = math.random(0, 200)
local randomZCoordinate = math.random(-75, 125)
local randomLocation = CFrame.new(randomXCoordinate, randomYCoordinate, randomZCoordinate)
local Character = player.Character or player.CharacterAdded:Wait()
Character:PivotTo(randomLocation)
end
end
local function teamJoin(Team)
Team.PlayerAdded:Connect(function()
local randomNumber = math.random(1, 100)
if randomNumber % 2 == 0 then -- If the number is divisible by 2, then...
randomizePositions() -- Activate the randomizePositions function!
end
end)
end
for _, Team in ipairs(Teams:GetTeams()) do
teamJoin(Team)
end
Teams.ChildAdded:Connect(teamJoin)
Edit #1 (November 1st, 2021) – Added LocalScript and Server Script examples for referencing a player’s Character.
Edit #2 (May 17th, 2022) – Updated the Roblox Developer Hub Documentation links throughout this post to the new Creator Documentation pages.
Because the Roblox Developer Hub will be superseded by Roblox’s Creator Documentation, I figured that it would be better to replace the links in this post sooner rather than later so that the main post wouldn’t have broken links in the far future.
Before
Player - Developer Hub Documentation
Player.Character - Developer Hub Documentation
CharacterAdded Event - Developer Hub Documentation
After
Player - Creator Documentation
Player.Character - Creator Documentation
CharacterAdded Event - Creator Documentation
Edit #3 (July 13th, 2022)
Added a greater variety of scripting examples (LocalScript Example #2 and what is currently Server Script Example #2 [the current Server Script Example #3 was the second one until I added the new example] in addition to referencing more relevant resources from Roblox’s Creator Documentation.
Can you use the local script example in a server script or would that not work
Without making any changes, both examples of LocalScript code would not work in a Server Script because the LocalPlayer (defined on the second line of code) is only accessible in LocalScripts.
Since LocalScripts run separately for every Player, there’s basically a unique copy of the LocalScript for every Player that joins the game. For example, by default, every time a player’s Character spawns into the Workspace, there’s an “Animate” LocalScript added to the Character that handles the Animations for that specific Character model.
The LocalPlayer object can be used to find out which player in the game the code is running for, which is especially useful for use cases like updating a ScreenGui Health bar so you make sure it’s:
Health
Workspace with the TweenService on the server).If you wanted the code in the second LocalScript example to work in a server script, you would need to reference the Player object in a different way, through something such as the PlayerAdded event, which fires when a player joins the game:
local Players = game:GetService("Players")
-- No longer referencing the LocalPlayer here because it cannot be accessed via a server script
local function updateHealthBar()
-- Example function
end
local function characterRespawned(Character)
updateHealthBar()
local Humanoid = Character:WaitForChild("Humanoid")
Humanoid.HealthChanged:Connect(updateHealthBar)
end
Players.PlayerAdded:Connect(function(player)
if player.Character then
characterRespawned(player.Character)
end
player.CharacterAdded:Connect(characterRespawned)
end)
However, as mentioned earlier, it’s important to keep in mind that client-sided updates and effects are recommended to be handled through LocalScripts, even if it’d be possible to update ScreenGuis and similar objects through server scripts.