We recommend you to focus on upgrading your base as a better defended base reduces the risk of you actually losing diamonds and gives you more storage and production, which can later on be used for all those sweet weapon upgrades. You may have to upgrade your diamond vault from the tab "Base defense", the first upgrade you see in here will upgrade your diamond vault. Also, remember that you will need diamonds to upgrade your diamond vault. You can collect diamonds every hour from your base or you can steal diamonds from other players via PVP(Player VS Player) games by winning against them.