I recently bought a BTC-wallet from my friend. It has a cold storage wallet ("Watch only") and a private key (which is a bunch of numbers and letters). How can I use it in Blockchain.info? How do I make (confirm?) transactions with it?
you question is not clear, what exactly have you bought?
anyhow if you have the private key you can import them in blockchain.info wallet in the Import/Export section. but if you bought it from someone else he/she also has the private key and so the ability to spend the bitcoins.