Ok, Oanda is not your 'average' FX broker. They do things a tad differently:
Most FX brokers have a minimum deposit of a few hundred, some a few thousand, but Oanda's is just $1. Can't do much with just $1, but Oanda says that's their minimum.
Lot size wise, most brokers follow standard lot sizes: $100k standard, $10k Mini, $1k Micro... but Oanda's minimum lot size is 1 unit of the base currency of the quote (basically $1, some exceptions.) Can't do much with $1 worth of exposure, but if you want to define your risk through lot size down to the penny, Oanda let's you do this (eg. opening a $34,123 sized position instead of having to round down to $34k at other brokers.)
Positions sizes are really exposure sizes. If you've traded FX futures, think contract specifications in size of the currency in question. Match the total exposure to a given currency, and you've matched the lot size you need to equal one contract in futures.
Oanda has no commissions, just the spread is paid. However a growing trend in the FX broker wold is to go spread + commissions. Brokers will pass on their spread feed from their liquidity providers for you to trade on, and make their cut in commissions instead of marking the spread up and passing it along. But as I said, Oanda is just spread, no commissions, so what you see is to the total cost up front.
Hope that helps. Oanda's a good choice. Just keep an eye on news release, they will widen the spread by a lot during the few seconds leading up to major releases and for a few minutes after... not the best for people who trade news releases.
