basically UGC (http://ugcleague.com) is a free league that's considered less serious (at least for 6v6) but is a good starting point for new players looking to get into competitive. the lowest division in north america is UGC iron, then steel, then silver, then gold, then plat. (some of those divisions might be gone now though i dont keep up with ugc)
ESEA (https://play.esea.net/) is a paid league where the more serious competitive players game, there's only 3 divs, open, IM (intermediate), and invite. some argue that players in open (usually low to mid open) are around the same skill level as gold/plat UGC 6s players. if that helps at all. although i would only go for a season or 2 of iron/steel/silver 6s and jump right to esea rather than climbing the UGC ladder.