There are several strategies to use when battling. I'm going to cover a few of these aspects!
One of the most important factors when you are building a team is the type balance. You don't want your whole team to be a bunch of Water Pokémon! You have to balance it all out. First of all, I suggest that you use the Marriland D/P Team Builder. In my opinion, you are able to have a little less diversity - meaning having 2 normal or water Pokémon. It all depends, though. If you have a team solely based on one task, such as a Rain Dance team, then you're going to have to compromise with only a few types in your team.
Another important factor is natures and abilities. Use them as your advantage! Abilites such as Serene Grace and Torrent can be useful. As for natures, you want to pick a nature that helps give your Pokémon the edge the most. For example, if you have a Blissey, you may want to first breed a Chansey with the Calm nature so it has some extra defense (quite deadly to have a Calm Blissey, might I add).
Now that you have your team "planned", I suggest you start with one Pokémon at a time. This shouldn't be too hard. I personally train my Pokémon by Exp. Shares and the Elite 4. Take it one step at a time. Train the first Pokémon to lv.50, the second to lv.50, and so on. Once you have reached a general area of uberness (xD), you are ready to set up its moves. I bet you're asking me: what? Yes. You may need to alternate your Pokémon's moves a bit.
While this may sound easy, it can actually be quite hard to get some of the moves you want for your Pokémon. What if it can only be purchased through the Gambling Prize Exchange place? If you bought the amount of tokens needed alone, it will cost well over 60,000. Hopefully it won't be too hard for you, though.
You finally have to EV train your Pokémon. This isn't hard, it can just be annoying and tedious. (I'll add more to this later)
Oh, finally! You have a decent team set up. Congratulations!
