What to do first when you get to TH 8?

1st builder - Lab
2nd builder - DE drill
3rd builder - Clan Castle
4th builder - Barrack
5th builder - new building (archers tower then wiz tower than ad then tesla)

Lab
DE Drill
All new buildings & walls up to TH7 max level
Clan Castle

Then work on splash defenses first, and go from there.

In my opinion, you don't really need to upgrade your barracks right away. I saved them for later.