I am very new to Bitcoin and just learning my way around my Bitcoin wallet, etc. Recently, in order to try to find out more about buying Bitcoin, I clicked on "Buy Bitcoin" in my wallet, but was immediately required to "enter Bitcoin address" before I could go any further. I don't know what to enter here, or how to find out. Can someone enlighten me, please ?
You have to download a Bitcoin wallet first. I suggest you to download Electrum, it's a lightweight wallet with easy UI. It will generate an address for you.
You can generate your bitcoin address via here: http://www.bitaddress.org -- make sure you write down or print out your generated private key. You enter your public key into your app you mentioned.