Well for starters Wiccas number one rule is to not harm. Would breaking up two people you never met really not cause any harm? Maybe not physical, but emotional.
Yes, breakup spells work. It won't work every time [no spell works 100% of the time] But you could if you know their location and have a picture or full name. The more personal you make the spell the better you can focus it and the spell to work. Breakup spells work by manipulation. If there's any doubt in the relationship the spell will have an easier time. However, if you're planing on breaking up two random people you met online I wouldn't hold your breath it would work.
If you just want to cast spells, please don't call yourself Wiccan, it's a religion, spells can be cast by anyone without a religion attached to it. If you are so new to Wicca you don't know The Rede I suggest picking up a beginner's guide written by a reputable Wiccan author before trying to cast spells. [Walk before you run after all]