Thank you very much, i was misunderstanding a couple of thinghs.
I've studied a little more the mosfets and i think i understand how to use them in some application now.
To be sure of that, could you confirm me that the following is a viable way of setting up a digital switch (for digital i mean it's controlled by two different input values) that must control a known load?
Im assuming the mosfet to be n-channel (V_th>0)
1) Find the current needed by the LOAD, I_L (for example by current = power/voltage or things like that depending on the load, i am thinking of a lamp right now)
2) Find the minimum gate voltage needed to trigger correctly the switch (allowing him not to saturate before the needed current is drawn).
In order to do this, I check the I_ds vs V_ds graph in the datasheet, and I find the first line who get to the right I_L before saturating (becoming flat, not dipendent by V_ds), and I pick the corresponding gate voltage value, V_gs.
3) I build a circuit like this one : http://tronixstuff.files.wordpress.com/2010/04/examp3p1.jpg
Where of course, instead of the npn transistor i have my beloved MOSFET,the load is the relay, and in place of the digital 2 pin I have any controlled voltage supply who can furnish the V_gs value i found at point 2)
4) I add all the stuff not related to the transistor usage that can improve the switch, like a pull to ground resistance for the gate or other things I don't know.
OK guys, will i blow up my house, or i will switch on the lamp?
Thank you in advance