A few things to consider:
1. Do you want a team that will compete for the Champions League (the Super Bowl of Europeon soccer)? If so your choices are down to about four teams at the moment, that will change unless you choose an English team. England has the most money in their domestic league (by far) but teams haven't been competitive in continental competitions recently. I don't see that changing any time soon.
2. Do you want to be respected by other soccer fans? If so don't choose Man U, Real Madrid, or Barcelona. Rooting for them is easy and most of their American fans are clueless. If you want a team that will win but won't be as shameful to root for look at the Juventus, Borussia Dortmund, and Tottenhams of the world.
3. Consider rooting for a team in a city you are likely to visit. Could you ever see yourself spending thousands of dollars to visit Liverpool or Leverkusen?
4. Don't root for Arsenal, 92% of their fans are convicted sex offenders. No joke, Osama Bin Laden was a huge Arsenal fan.
5. English teams are the easiest to follow in America, all Premiere League games are available on the NBC sports app and you can easily find their games on TV. Like I said, don't expect them to win games outside of their domestic league but it makes being a fan easier.
6. Again, DO NOT ROOT FOR ARSENAL. I can't be any clearer on this. Just say no.