OR Gate

You better put a resistence, before the LED

He's right. You better, or else you will waste 1.50 dollars of a green LED.

Or a logic gate at 4v

I didn't knew that ! Thanks for advice.

You also only need one power supply, with the two switches wired parallel off of it. When the light from an LED looks like an X, you blew it up. You'll need a resistor. Also, standard logic gates like they're input signals to be between 4.7V and 5V. There are certain families of them designed to accept input anywhere from 2V to 5V, though. I'm sure there might be some applications where a circuit would require different input and output voltages, but generally, if you're using 3.3V as inputs from your circuit, you'll probably want you logic to output 3.3V as well. It works though, and you've got the basic idea of the logic down.

Resistor of how much ohms should I use ?

Ohm's Law, my friend. R = V / I. R is resistance, V is voltage, I is current. Your supply is 5V. Your LED is 2V. So you have a difference of 3V. The LED will be using 20mA. So, R = 3V / .02A, R = 150.

I knew Ohm's law just didn't knew how to use it here.

Thanks @rbrtkurtz

No problem.

Good ol ohms law equation. Normally when look at some scheme or think about some novel way to solve particular problem this equation is present non-stop in mind constantly made some simple calculus even without thinking bout it its somewhat automatic. The same KL equations it's somewhat basic first line tool whenever u consider some design solutions. But AC ohm law uhmmmmm....... where i was put my engi calculator??? lol :-)