I tried to use jakdojade and it made it a little clear
Often jakdojade makes things worse!
1. Can i buy tickets for the bus which is taking me to train from Modlin, in Modlin? Do they accept euros?
Yes, you buy them on the bus. They don't accept Euros and they don't carry much change. Get some zlotych from the cash machine inside and buy a can of pop at the shop so you've got some coins or a small note.
2. Is it hard to find the right train track in Wschodnia, and do people who sell tickets speak english, usually?
It's easy now - Wschodnia has been modernised and is easy to use. I doubt many of the staff speak English, so write 'Siedlce' on a bit of paper if you're unsure of the pronunciation. You could also write 'bilet w jedną stronę', meaning 'one-way ticket'. No need to get a return ticket and it isn't expensive.
. If i take money from an ATM, will i receive Zloty? (I live in Italy, so Euro is my currency) I'd like to avoid kantor in airport because i know they have some bad change rate.
Yes - you'll get zlotych.