Levels and Badges
If you have other games from this generation you may want to transfer in some of your higher level Pokemon to make life a little easier -- if you do, and also keep this in mind when leveling the ones you catch in the game, always remember that the badges you have are what dictate what level Pokemon you can successfully issue commands to!
The Badges in the game each bestow specific abilities:
Stone Badge: The power of all Pokemon increases a little. You can now use the move Cut any time.
Knuckle Badge: Pokemon up to level 30 obey you. You can now use the move Flash any time.
Dynamo Badge: The speed of all Pokemon increases a little. You can use the move Rock Smash any time.
Heat Badge: Pokemon up to level 50 obey you. You can use the move Strength any time.
Balance Badge: The defense of all Pokemon increases a little. You use the move Surf any time.
Feather Badge: Pokemon up to level 70 obey you. You can use the move Fly any time.
Mind Badge: Your Pokemon special attack and defense are stronger. You to use the move Diving any time.
Rain Badge: All Pokemon regardless of their level will obey you. You can use the move Waterfall any time.
When it says that Pokemon up to Level X will obey you, that includes any Pokemon that you have traded into the game from another cartridge or obtained in an in-game trade. Bear in mind that with no badges you can fully control Pokemon up to around Level 18 but after that they may not obey your commands until you have obtained the first two badges. Once you have the first two badges then as it says, any Pokemon up to Level 30 will obey you. Also note that with no badges, there is a chance that traded Pokemon will refuse to obey you.
As your adventure begins in Littleroot Town, try to remember to take your time and really enjoy the game! You have plenty of time before Black and White are released, so use that time to savor what is widely viewed as the best story in any of the games after the saga of Ash in the original Red/Blue/Green!