You can find a tutorial about how to register on the channel here: Registration Tutorial: MSP/PALnet.
After you registered with the bots, you can use the #upvote channel to request free upvotes to one of your (or some else's) posts every 12 hours.
It's very simple. Just write "$upvote" and after that the link of your Steemit post.
You will see how other Steemians are doing this. You can also use the "?upvote" command (even after you used the "$upvote" command). You can use it before you use the "$upvote" command as well. If you (also) use the command with the question mark, then @dorabot will (also) upvote your post. That's also free of charge of course, but most of the people are using the command only with the "$" sign.
If you want to promote your posts, you can use the #postpromotion channel. As far as I know, there is no time limit on this service and it's also free.