How can we know our "from" address in advance? This website ask me to enter the address FROM where I would be sending BTCs.
But every time a send some BTC it seems to be sent out from a different address...
Should I use the one I always see when I say I want to "receive" some BTC? Is it my "main" address?
There is no from address in Bitcoin.
https://en.bitcoin.it/wiki/From_address

Each transaction reference an unspent output (UTXO) that was sent to you in a previous transaction.

About the address, you need to generate a new one for each payment:
https://en.bitcoin.it/wiki/Address