Use your initial free gems to buy workshops. Don't buy immunity shields as they're a waste of time at the start. The more workshops you have, the more troops and buildings you can upgrade simultaneously. There's nothing worse than sitting on a pile of coins and/or stones waiting for an upgrade to complete because you're a nice target to be picked off.
And yes - decorations are largely a waste of gems/money.
Another tip is to learn the different ways your troops attack and how defensive weapons defend against you. Use that knowlegde to your advantage by initial placement of your troops when raiding.
Watch replays of raids against you to learn troop and weapon behaviour. Subtly change your defensive position if attacked so that if the same raider comes along later, they can't use the same attack strategy.