Hi, Alex, welcome to Steemit! 1) To vote for witnesses, click on the "triple bar" icon on the upper right corner of your page and select "Witnesses" from the menu that appears. 2) Yes, you can certainly just add comments if you don't want to make posts. Posts tend to be the most profitable, but starting out with comments is a good way to get familiar with the platform. 3) The error you're getting is because you're running out of Steem Power, so you need more to vote more often. We just noticed this issue yesterday for some new accounts, so the witnesses are pushing now for more Steem Power to be allocated to new accounts so that they will be able to vote more often with their initial allocation.