1) What lights a LED up? Voltage or current?

2) What does a resistor limit? Voltage or current? I mean, if I don't put a resistor in my circuit the LED bursts down: why? Too much volts or amperes?

This should get you started as far as answering questions 1 and 2:

http://www.sparkfun.com/tutorials/2193) What remains constant in a circuit? Voltage or current? I mean, what is the same at the start and at the end of a circuit? Volts or amperes?

It depends on your circuit. I asume that you are familiar with series and parallel circuits. In a series circuit, current is constant (assuming none of your resistors change values due to heat or other factors) and voltage will change from place to place. If you have a battery connected to two resistors (motors, resistors, anything really...) then you can use Ohm's Law to calculate everything about the circuit. Let's say we have a nine volt battery and two 50Ω resistors.

Ohm's Law: Voltage = Current x Resistance

We know Voltage (before the battery starts to die) is 9V. We also know the value of both of our resistors in the circuit. In series, resistors simply add their resistances to get a total. 50Ω + 50Ω = 100Ω, so now we have:

9V = I x 100Ω

To find current, divide each side by R:

9V/100Ω = (I(100Ω))/100Ω

0.09 = I

So the current is 0.09A, or 90mA. When working with many things currents will be listed in mA, so it's worth it to convert.

To find the voltage across a certain resistor you already know that the current (constant in series circuits) is 90mA, and you know the resistance of the part, here 50Ω. So:

V = 0.09A x 50Ω

V = 4.5V

The most important thing to get out of this is Ohm's Law. It is used everywhere and flipping it around, you can get almost whatever you want to know.

As far as parallel circuits go, you still use Ohm's law, but there are some important differences. Look here and see if you can get them figured out.

http://www.allaboutcircuits.com/vol_1/chpt_5/3.htmlHere is another good source:

http://www.electronics-tutorials.ws/dccircuits/dcp_2.html4) How should I put resistors when I have more than one LED?

It will depend on your circuit. If all of the LEDs are coming on at once in series, you could get away with a single resistor calculated to supply the correct amount of current. (This is referred to as a current limiting resistor.)

If the lights are independent of each other then you will need a resistor for each LED.

5) How should I put resistors when I have series or parallel circuits?

Once you understand the various parts of series and parallel circuits, this will become easy for you to decide.

Hope that helps!