You enter Electro Gym.  As you enter you notice 3 Junior Trainers waiting for you.  If you need a little more exp before fighting the leader, you can challenge these three.
Fight Scott (Lv 12)
Fight Jacky (Lv 13)
Fight Kyle (Lv 14)

After you defeat them, you follow a long corridor that leads to the Gym Leader's room.  You then spot the leader, StevenW.

"Aha. Another challenge," he replies when he notices your presence.  You ready your best Ground Pokemon for this fight.  As if he is able to read your mind, he replies, "Hahaha fool.  You actually think that my team consists only of Electric types?  If you plan on defeating me, you better have a good balanced Pokemon team!"
Fight StevenW for the Electro Badge!
If you think you're still not ready, you can leave the gym to get more practice.