Can i find values of variable in a table using a string?

You use keys. Keys can be almost anything--userdata, strings, numbers, and even booleans. However, you probably shouldn't rely on userdata for a key.

Keys aren't specifically for tables however, they're like fancy dot notation (dot notation being Part.Mesh.Scale).

You can use keys in such a way to index properties of an Instance:

local IN = Instance.new("Part");
IN["BrickColor"] = BrickColor.new("Toothpaste");
IN.Parent = workspace;

Or, you can use keys in a way to index an Instance's children:

local IN = Instance.new("Part");
local NewIN = Instance.new("Part", IN);
NewIN.Name = "New Instance";
IN.Parent = workspace;

print(IN['New Instance'].Size); --2, 4, 1 (or something like that)

To index a table using a key, you use the square brackets (note, SQUARE brackets, not parenthesis.)

For example:

Using a Key to make indices within a table:

local Table = {};
local value = "Hello, World!"

Table['key'] = value;

local MyValue = Table['key'];

Generating a table using hardcoded indices, then getting the value.

local Table = {
	one = true;
	two = false;
	three = "three";
}

local OneValue, TwoValue, ThreeValue = Table['one'], Table['two'], Table['three'];
print(OneValue, TwoValue, ThreeValue) -->> true, false, three

Using keys within a table itself:

local Table = {
	["Thwack"] = {
		Damage = 45;
		Description = "THWACK (nintendo please nerf hero i beg of you)";
	};
	["Whack"] = {
		Damage = 25;
		Description = "hero doesn't deserve this power"
	}

	["Inside of Party"] = true;
};


print(Table.Thwack, Table['Thwack']); -- table 0xMemoryAddress table 0xMemoryAddress
print(Table.Whack, Table['Whack']); -- table 0xMemoryAddress table 0xMemoryAddress
pcall(function()
	print(Table.Inside Of Party) --errors
end);
print(Table["Inside of Party"]); --true

Resources

Be sure to look up resources to further develop your knowledge about tables. Tables are a major contender for one of the most useful basic abilities for a language.

Lua-Users: http://lua-users.org/wiki/TablesTutorial