Having used for several years, I can say that IGIndex have a good range of binary bets, will take reasonable stakes and are fundamentally honest.
CityIndex and CapitalSpreads offer a smaller range of binaries, I have used only a little but without any problems.
ChoiceOdds are to be avoided, they are basically crooks - if you start to show any sign of beating them then they will, in effect, start stealing from you by putting you on big delays, accepting the bets that start losing and and rejecting the ones that start winning. This behaviour is discussed in these threads:
Gives you a tutorial (House price Betting) but the same applies to other markets such as Gold or Oil.
Basically, set the bounds between which you think the market will close, set your stake and place your bet.
To trade with real money you need to enter (securely) your Visa or Mastercard details (if you are UK based) and yo can trade with real money. You pay for each bet as you make them.