When you signup on steemit.com you get some free Steem Power as a start, it should be the equivalent of 10 Steem Dollars I think, so that is why you already have some in your account's wallet.
When you get in the witness queue list everything is still happening automatic like the mining process, there is no need to do anything manually.
Steem mining is done using the CPU of the computer, so if you are mining on a GPU(s) on the same system and max out the processor load with mining threads it can result in drop of the GPU mining hashrate. You can lower the number of mining threads or lower the priority of the steemd.com process and increase the priority for the other GPU mining process from the task manager.