Okay Justino here's the straight dope.
The chances of you selling a spec tv script are somewhere in the neighborhood of zero. TV series all have staff writers - and each episode of a show for any given season is often (not always) part of an ongoing story arc that plays out throughout the season. At the beginning of season, the producers generally know what the overall arc will be (i.e. last season on Friends - Monica & Chandler's baby arc). Most episodes deal with these ongoing arcs in some way. As an outsider, you are not privy to the overall arc.
Sometimes a series will contract with a non-staff writer to write an episode and this is where the spec comes into play.
TV specs are basically show scripts - they show your ability. If you write a spec for Everybody Loves Raymond - the chances of selling that script to Everybody Loves Raymond are basically non existent - but if your write a good spec for Everybody Loves Raymond, the producers of Joey may let you pitch them an idea. And they might hire you to write an epidsode of Joey - which could lead to other assignment jobs - which could lead to a staff position.
You should pick shows that have been on long enough for you to have a good understanding of the characters, tone, etc. But no shows that are too old. You also don't want to pick shows that are too hot. Because everyone is writing specs for the hotter shows and producers get tired of reading them. For example a couple years ago everybody had a South Park spec.
It is best to have a couple of show scripts (two or three) you need to really analyze the shows, to see how many scenes they have, how long the scenes are, etc. Each show has its own rhythm. Seinfeld had 15 or 20 short scenes per ep. others have 8 or 9.
There are also different screenplay formats for drama, comedy shot on tape, comedy shot on film, etc. For example sitcoms shot on tape are doublespaced and descriptions are all caps - most of the screenplay formatting software have different templates for different types or in some cases specific shows.
TV producers are even more resistant than film producers to accept unsolicted manuscripts - partly because they have a staff of writers so they don't need outside writers, but mostly because with the amount of story ideas they need (more than 20 each season) there is too great a chance that someone will submit a story idea similar to one that ends up being used and then they have to deal with possible lawsuits.
So an agent is really a necessity when seeking television work.
There may be some shows - maybe some cable shows or whatever that will accept unsolicited scripts - but what I described above is the way things work in TV.
As for creating your own pilot - it may happen as a fluke once in a blue moon - but generally speaking it is not worth the time or energy to try. Networks and major Prod co's want experienced writers as show runners and producers. The demands of a weekly series are such that it is too big a gamble to give someone who is unproven a shot. If you know an experienced staff writer or producer you could team up with that's one thing. But no one is going to take a series pitch meeting with someone who has no experience in TV.