Welcome to the land of Star Trek, Louis
, your life will never be the same (though I'm not going to say if it will get better or worse
Here's my read on the series:
TOS: Not a bad series, some episodes have aged better than others, but it was overall the most creative and unique of the five. When you look at some of the stories the told, you can tell that they would have never even been considered nowadays, and those stories make it pretty entertaining. It is also the funnest of the five.
TNG: Like TOS, it is primarily a show about one ship buzzing through the galaxy meeting new people. It's a good series, and like others have said, probably a pretty good gateway series.
DS9: My favorite of the bunch. A story-arc that lasts nearly the whole series. Many of the episodes are top-notch material, you get to know and become attached to the characters, and it is overall highly entertaining.
VOY: This is my least favorite of the 5. As others have said, it was "Lost in Space" Trek style. It was a good concept, but wasn't executed to it's full potential. There simply wasn't enough of a sense that they were lost and on their own... it was TNG without meeting other Starfleet ships. It had a few jems, but for the most part, it was just kinda meh.
ENT: I was personlly fond of this series, which it seems I a wee bit of a minority on. I thought most of the characters were very interesting, though at times, it didn't quite give it enough of a prequel feel. The first season was good, season 2 was a bit of a let down, but season 3, and especially 4, really hit their stride and started producing excellent shows.
For the movies, I think it's important to watch the first 7 in order (Motion Picture through Generations) because they are a sequential story, and movies 2-4 are actually a trilogy of sorts. The last 3 aren't as important to go in order on, but you may as well. On a final note, I will agree with the other posters: if you want your own experience on watching these, avoid this place, or at leas the Trek section until you finish. We are very opinionated, and like spoiling things. Have fun!