Striaton City

The first thing you should do when you get to Striaton City is heal your Pokemon and save your game. After that, right near the entrance, you can actually break through the west part to get yourself an X Speed, which may come in handy (or may not).

Go inside of the first building, entering from the side (yes, you can do that now!). Talk to the man inside and he'll give you a Great Ball, which is just great at catching Pokemon. It's much more effective than a normal Poke Ball, so save it for something special (although don't wait too long, since you'll be able to buy 'em later in the game).

If you go behind those buildings, you'll find a shady character who will give you a Dusk Ball if you talk to him. That's actually great, because it is an extremely effective ball, but only if used at night or in other dark places.

There's another Great Ball hiding in the eastern side of town, by the streams and gardens. It's hiding behind some trees in the south. It can be a little tricky to spot, but it's there, don't worry.

Anyway, before you take on the Gym, you should really go over the Pokemon School. Cheren's waiting there, after all! If you talk to one of the little boys there, he'll want to quiz you. Just answer the top answer (Antidote) and then the bottom answer (Awakening) and he'll give you a free Full Heal, so you can cure any of those status conditions.

Now, go ahead and talk to Cheren and he'll ask if you'd like to battle him, since he's using the type that the Gym Leader will use against you. You should definitely take him up on it, even though it might be a tad tricky.

Cheren is much more difficult to take down due to his type advantage over your starter Pokemon, so hopefully you have another Pokemon trained that can help take it out. Nothing you have at this point will be overly effective against it, so don't worry too much about it, and your starter can likely do pretty well if it is fairly high level. Beware, though, because his starter has an Oran Berry attached, which will heal it up when it gets weak.

After beating him, he'll give you an Oran Berry to help you out. That very well may come in handy against your Gym Leader battle, but for now, you should leave it unequipped, because it'd be a shame to use it up too soon!

Now that you've beaten Cheren, you can head to the Gym to the right of the Pokemon Center. The guy who was blocking it earlier is gone and replaced with someone else instead: the Gym Leader you are about to face! Talk to him and he'll go inside right away to get ready for you.

Before you do, though, there's a new Pokemon you'll definitely want to pick up to make your battle against the Gym Leader much easier, since the Gym Leader is going to use the type that you're most weak to. Head on to the east of town, over to the Dream Ruins! Just make sure you have at least one empty slot in your party before you leave (and healing would be great, too!).