@knircky has already answered your question very well. I would just like to give you some practical tips and tricks on how to find posts to vote. That is of course if you are just after curation rewards and not looking at the quality of the posts, which should be only proper reason to upvote.

Few practical tips & tricks:

• Use SteemWorld.org to monitor bot activity. Replace your name with bot name and look for "transfers in". The posts, on which their authors placed high bids and have previously zero or just few votes, are the posts you should upvote. You should consider the time frame of 0-30 minutes (0 - 15 after HF20) as explained by @knircky. You should normally have enough time to place your vote before bid bots.
• Search for hot posts with quality content, who have just a couple of upvotes and are not older then 30 minutes (it will be 15 minutes after HF20 is implemented). You can do this search by tags (i.e. travel, food, photography, crypto, etc.).
• When it comes to commenting rewards (this go under author rewards, not curation rewards), concentrate on placing meaningful comments to posts of those authors, who are actually rewarding such meaningful comments with their upvotes.

As @knircky explained your actual curation reward also depends on your SP and voting power, so with your current SP you can not expect very high curation rewards.