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Start putting some in Steem Power. That determines how much bandwidth and voting power you have. Its good to have at least 15 steem power. Steem Dollars is what you use to spend on upvoting and also to transfer to other people in the network. Whatever you put in Steem Power does get locked up for 3 months. But you don't lose it and will probably have gained value after the 90 days is up.