How can I find out the ID of a transaction I just made?
I sent bitcoin to a site and they are requesting me to send the transaction ID to them.
How do I find it out?
Some wallets have a function called something live "View on block explorer". For Electrum, you can see it if you right-click a transaction.
This will take you to a website on which you find your transaction ID. I drew a red ellipse around it so you know which number it is.
If your wallet doesn't have that functionality or you don't find it, simply copy the address you used to send the money and paste it into the search field on https://blockchain.info. You'll get to a page showing you all transactions that address has been involved in.
Green arrows mean that that address gained money, red arrows mean that that address lost money in the transaction.