If you poke around you should be able to find a Glock Armorer's manual online for download.
Read it and you will understand which parts you want to replace.
There are two parts which can vary the trigger pull weight on the Glock.
One is the trigger spring, the other is the connector.
The connector is available in three styles, the 3 lb and the 5 lb and the 8 lb.
The springs are available in three weights, stock, New York and New York+
With a stock spring, the connectors will give pulls that are about 3.5 lbs, 5.5lbs and about 8lbs.
With the New York spring, the 3 & 5 lb connectors will give pulls that are about 5.5lbs about 8-9lbs.
With the New York + spring, the 3 and 5 lb connectors will give pulls that are about 6-7 lbs and 9-11 lbs.
Glock states that you should never use the New York springs with the 8lb connector.
I recommend that you start with the stock coil spring and a 5lb connector. That's the original weight, and I think that you won't have any troubles with it.
You could also go with the New York spring with the 3lb connector. That also gives a pull that is about 5lbs, but it's got a different feel that some folks like.