New rear derailleur fitted, fixed to one gear now

[Luke]

Recently on a bike ride I made a gear change and my rear derailleur broke and 'exploded' into about 10 pieces. I purchased a new one and my father fitted it on. Now, I'm fixed to one gear (2 i think) and I can't really ride my bike.
How can I fix my gears so I can use them all?

[Alex Ramon]

Undo the shift cable, put the shifter in the highest gear, and loosen the 'H' limit screw until the chain is sitting centered on the smallest rear sprocket. Then tighten the cable and try setting it up as explained in this video:
http://bikeride.com/adjust-rear-derailleur/