While your tribe is still small it is important to protect your Town Center from raiding barbarians and other attackers, so while you have enough Gold, it is recommended to start building towers and barracks for your defenses. With barracks, you may have the option to train soldiers to be part of your campaign against other tribes.
Like most things in the game, training soldiers and troops takes time--the stronger or more advanced the soldier is, the longer it takes to train them. If waiting is not your thing, or if you need your troops immediately, then you may spend a few Crowns to speed up the training.
Each soldier you pick occupies a certain amount of troop space. In order to increase troop space, you will have to upgrade your barracks from time to time. Aside from upgrading your barracks, upgrading your soldiers or troops are also essential by upgrading your Blacksmith.
At this time, you may be attacked by invading barbarians in the area. To get even, you may train more troops to attack those barbarians and engage in your first battle.