I hope you don't mind that I use some grammar terms that are a bit advanced, but I think that they will be really useful for you, not just for understanding से (se) but also other postpositions. से (se) can be used to mark at least two "cases" (functions or uses of a word): ablative and instrumental.

1.) Ablative (when the marked word is an origin from which an action takes place).

For example: मैं दिल्ली से हूँ। (maĩ dillī se hū̃), "I'm from Dehli".

2.) Instrumental (the marked word is a tool, a material, a person or other thing using which or with which something happens).

Example a.) मैं हाथों से अपने पैर धोता हूँ। (maĩ hāthõ se apne pair dhotā hū̃), "I wash my feet with my hands / I use my hands to wash my feet".

Example b.) यह मेज़ लकड़ी से बनी है। (yah mez lakṛī se banī hai), "This table is made of/from wood".

Example c.) मैं तुमसे बात करता हूँ। (maĩ tumse bāt kartā hū̃), "I'm speaking with/to you".

Example d.) मैंने तुमसे मेज़ बनवाई। (maĩne tumse mez banvāī), "I had you make a table"
(In this last example we have the causative verb बनवाना/banvānā, and से/se marks who really performs the action of table-making).

--I'm probably leaving a couple of things out but I don't want to make it more complicated than it is. The use of one postposition over another is not always so logical. I think that the best way to learn the use of postpositions is to hear them in use and to repeat them. :)

Main ghar se ja raha hoon. ( I am going from home).

Main tum se baat nahi karunga.  (I wont talk to you).

Tum kal se school jaoge.  ( You will go to school from tomorrow).