Basically, for a minnow SP is key. SBD may be useful to buy into a voting bot, like @randowhale or so, but its no use to save up bigger amounts of it. Use it to buy Steem that lets you power up your SP.
What the best method for the converting is... hard to say. The exchange rate vary a lot. You can trust the Steemit internal rate, or try your luck at the market. I usually suck with this, so I better not try to give you advice on this... :)