Downloaded bitcoin core and it says synchronising with network, how do I get it to download the blockchain? I downloaded Bitcoin core today, and my got an address for my friend to send me some bitcoins. He sent them, and I didn't realise I should have waited until the blockchain downloaded, and this can take some time. My problem is, at the bottom (iOS) it just says 'synchronizing with network' and that's it? The bar is empty (not green or anything) and it hasn't downloaded anything yet. Could someone please help me!! I'm extremely worried I've lost the money my friend sent :(
You have not lost the money, "synchronizing with the network" means that your Bitcoin Core is downloading the blockchain! 

This may take a few hours or days depending on the computational power of your computer. If you don't need them immediately, you can just wait until the synchronization is finished which is what I would suggest that you should do. 

If you are anxious to check whether the bitcoins were transferred, you can look up your address (that it was sent to) on one of the block explorers.