After defeating Red, go to Silph co. and he will be at the main floor. Steven will offer to give you a third generation starter (treecko, torchic or mudkip).
you dont.
No you can't
You can't battle Steven in Pokemon HeartGold.
You can't battle Steven in Pokemon HeartGold.
You can't battle Steven in Pokemon SoulSilver or HeartGold.
You get it from Steven way later in the storyline
It is given to by Steven Stone after beating Red.
trade that steven dude for an evolved pineco
you've beaten the game gratz
you get beldum.
Silph.Co(Kanto, postgame)
you cant