Will it only be random output to mix in a transaction or can i chose my own?
In theory, you could choose yourself if the software supported it but I don't see much benefit in doing so.
In practice, the current monero-wallet-cli has 2 different transaction building algorithms depending on which transfer command is invoked. The 2 commands for the monero-wallet-cli are
transfer-original. I'm not familiar with how they decide which outputs to pick, though.