When you have low SP (Steem Power), the best thing to do is upvote what you're sure will make it, otherwise you will divide your votes between all the posts and replies. This isn't good because eventually your upvote won't even be counted, as if it was nothing. It recovers completely in a 24-hour period. So, from my understanding, voting around twice each day on low SP maximizes your voting power.

Buying SP is wise if you want Steem to grow, as it's literally a long term investment, because you cannot take out your SP entirely for a minimum of 1 year. Please think of it carefully.