There are several competitive TF2 Leagues in North America. Choosing the right one is important, as each has their own rules, match dates, and quirks.

RGL.gg
RGL, or Recharge Gaming League, is a relatively new league to the competitive scene. Created and run by Sigafoo, a long-time platinum engineer, RGL popularized the Prolander format, and has recently begun to run seasons of Highlander.

Pros
Match servers supplied by the league
Fast Support
Free Registration for lower divisions
Prizepool

Cons
Less well known in the community
No 6s or 4s
HL has same default match time/date as UGC


UGC
UGC is the longtime "standard" TF2 league. Created in 2002, UGC was originally a TF Classic league,and is now the biggest TF2 league. This behemoth of a league has three seasons of Highlander, 6v6, and 4v4 a year.

Pros
Well recognized; everybody knows the divisions
Free Registration
Large community

Cons
Servers are not provided
No prizepool
Support can be slow
Sandbagging (experienced players in low divisions) is common
Medals are generally very slow to be delivered

ESEA
ESEA is a Counter Strike league that also runs a 6s tournament. This makes it very different to other leagues. The lowest division (open) is very large, meaning that often very new players go up against experienced ones. It is not recommended as somebody's first league, but is still on this list because of it's prize pool and prevalence in the 6s community.

Pros
Large prize pool
Large yearly LAN tournament
Well known esports league
Also a Counter-Strike League; "premium" subscription transfers
Division structure similar to ETF2L & other esports
Servers provided

Cons
Only 6s
Open division is large and diverse
Fairly large registration fee
ESEA client is known to be buggy

Hopefully know you're better equipped to find a league top play in. Bear in mind that there are other smaller leagues in North America, such as TFCL that you might also be interested in. More information is available on http://comp.tf/wiki/Main_Page