The first woman you see will give you TM 94, Rock Smash. It is a Fighting-type move that is super-effective against the Normal-type Pokemon that the gym leader in Nacrene City uses. The best type of Pokemon to use this attack is a Fighting-type Pokemon, so let's keep going until we find a patch of grass to catch one. If you chose Tepig and it has evolved into an Pignite, it is now a Fire and Fighting-type, so teaching it Rock Smash would be good for the second gym battle. Keep walking west, and then south to find a woman in pink who will only battle if you talk to her. She has a Munna at a high level, so be prepared.
Being a nurse, she will heal your Pokemon for you and she will now heal your Pokemon each time you talk to her. If you walk between the fence and the trees you will eventually find an Ether. South of the Nurse is a female Preschooler.
If you press A at the big rock here, you will get a Star Piece which can be sold for a lot of money at a Poke Mart. You can get a Star Piece here once every day. Walk down the second set of stairs and there will be a Net Ball. It works best on Water and Bug Pokemon, otherwise it is just like a regular Poke Ball.
If you didn't start with Tepig, you most likely don't have a Fighting Pokemon yet. The grass around here has three different Fighting Pokemon to catch: Timburr, Throh, and Sawk. Throh can be found normally in White, and only in shaking grass in Black. Sawk can be found normally in Black, and only in shaking grass in White. The best Pokemon to catch and train for the second gym battle is Sawk, because it has the highest attack stat and pretty good speed. If you teach it Rock Smash with the TM 94 you got earlier, it will do a lot of damage to Normal-type Pokemon in the second gym. When you're ready for the second Gym head back to Nacrene City.