On your way back onto Route 19, Alder will jump down from his cliff once again and give you 5 Oran Berries, which you can have your Pokemon hold in order to heal 10 HP when it gets low on health. That can make a huge difference at this point in the game, so be sure to equip them before taking on the Gym Leader, although you may want to save them until after you've beaten the Trainers in the Gym.
The Gym can be found to the west of the Pokemon Center. Enter it and you'll see it's some sort of school. Before you go rushing in, though, be sure to talk to some of the kids around; one of them will give you an X Defend.
You'll have to face off against Cheren's two school kids before you can battle him; the first one has a Patrat L9 and a Lillipup L9. The second has a Lillipup L9 and a Patrat L9. After defeating them, I recommend you head back to the Pokemon Center to heal your Pokemon back up to full before taking on Cheren.When you enter the back part of the Gym, you'll be greeted by a man who introduces himself as Clyde. He's the Gym Guide and will give you a Fresh Water in each Gym if you talk to him (in several he'll talk to you automatically). That's great, because it heals a whopping 50 HP, which is way more than you probably have at this point.
I recommend being at least level 13 before taking him on if you're only using your starter Pokemon, or to have two or three Pokemon at level 11 if you're using multiple Pokemon. While it is possible for you to catch a Riolu before taking on the Gym, it doesn't learn its first Fighting-type attack until level 15, making it a rather impractical option.Both his Patrat and his Lillipup know the move Work Up, which raises their Attack and Special Attack by one stage. Be wary, because if he uses this more than once, it can spell disaster for your team, as his attacks become very powerful and there is little you can do to counter it! If you have Pokemon that can Growl, you can use that to counteract the effects of Work Up. Using an X Attack or Snivy's Growth early on in the fight can be a good strategy to increase your damage output, but it does come at a risk, giving Patrat an opportunity to use Work Up. Other than that, their only real attacks are Tackle, so as long as you can handle Work Up, you're good.
After beating Cheren, he'll reward you with the Basic Badge, which ensures traded Pokemon up to level 20 obey you without worry, and he also gives you TM83 (Work Up). You can teach any Patrat or Lillipup you have — among other Pokemon — the move his used against you. It can be a helpful move for longer fights, but in most shorter battles, you're better off just using your ordinary attacks.
Now that you've beaten him, head on outside of the Gym. Bianca will be waiting there for you! She'll give you TM27 (Return), which is the light side of the Frustration TM you got earlier. Its power increases depending on how happy your Pokemon is with you. It may not do all that much right now, but later on, it'll be a staple Normal-type attack that has a maximum of 102 power when your Pokemon is at maximum happiness with you.
Now that you have the C-Gear, you can trade and battle with other players, even if it's with the original Pokemon Black and White games! That's right, you can trade over any of your Pokemon Black or White Pokemon at this point. You can also visit the Entralink by selecting the Wireless option and then tapping the option on the left for the Entralink. This is where you can interact with other players or games in the Entralink (over local wireless only) or where you can accept things called Funfest Missions to do tasks for NPCs, apparently.Cheren then pops out of the Gym and both he and Bianca exchange Xtransceiver numbers with you. All of a sudden, your own Xtransceiver rings, and it's Professor Juniper! She talks to you, Cheren, and Bianca in a four-way video call. So the three of them talk for a bit before ending the call, and then Hugh shows up, ready to take on the Gym Leader. Perhaps a little too ready. Lastly, Bianca gives you the C-Gear, which occupies your bottom screen. It'll ask you if you want to turn it on at first — unless you plan on trading or playing with other people playing Pokemon Black/White or Black 2/White 2, you can leave it off to save battery life, but it is pretty neat.
Anyway, now that you've got that, it's time to heal up your Pokemon and then head back on over to Route 20!