What do you do after beating Lance in Pokemon HeartGold?

go back to new bark town then go to the indigo plateau

First answer is wrong you do that after the 8th gym.

What you do is once you have 7 badges head to Lake Rage to catch the Red Gyarados if you haven't already once you do that return to Mahogany town where you go inside that house with person who sells you things Lance will be there and fights with you it should be self explanitory there. :)

Actually you have to go to Goldenrod City. When you get there, you go underground, there will be a Team Rocket Grunt there, but they will give you a Team Rocket outfit. Then you go to the radio tower and I think you're okay from there.