step away peasants, i am the black and white expert here
You really shouldn't play BlazeBlack/VoltWhite first. Aside from being better than the originals, they are really, really hard.
Onto the originals, though.
Yes, you should play BW before BW2 because as stated, a lot of things in BW2 will happen that you won't really understand unless you've played the first games.
Picking the starters, once again, is no easy choice. Snivy is the fastest out of the three, but i prefer using it in BW2 due to its expanded movepool thanks to Move Tutors, Serperior being able to be tutored various moves like Aqua Tail, Dragon Pulse, Iron Tail, and even Outrage. Combine some of that with Coil and you have yourself a fast, lethal sweeper.
Moving on to Oshawott. Samurott might actually look like a super strong physical sweeper, but his Sp. Atk is actually bigger than his Attack. But then again, Samurott kind of goes both ways. With 100 Attack and 108 Sp. Attack, it can easily work with both Physical and Special moves. You'll have to hold on to Razor Shell/Water Pulse a lot until you get Surf, which is obtained only after getting the 6th badge. Your Samurott should be level 36-41 somewhere until then. About move sets, i can't really help you. I suggest you get Surf/Aqua Tail, Blizzard and Megahorn on it. The last move can be anything you want.
On to Tepig, everyone's least favorite Gen 5 starter. Guys, look. Emboar isn't bad. People always think "ugh, another Fire/Fighting type starter, he must suck", but that's not true. Yes, i agree he's the worst Fire/Fighting starter, but he works quite fine for Gen 5. With its 123 Attack and 100 Sp. Atk, it works better than Samurott as a Mixed Sweeper, although i prefer using only Physical moves. All my Emboars are equipped with a set of Head Smash, Wild Charge, Flare Blitz and Hammer Arm. Flare Blitz is obtained at level 62, so you'll have to settle with either Heat Crash or Flamethrower. Head Charge is learned at level 50, your Pokemon will probably be around that level nearing the end of the game. Hammer Arm can be learned via Move Reminder in Mistralton City.
Give me an idea of what kind of team you want and i'll help you with it.