First raids
Raids steal resources and bounce. Attacks fight to the death and can take buildings down. Your first week should be raids, mostly on oases, until you understand carry capacity and crannies.
Send screen
Open the Rally Point and pick the send order: reinforcement, normal attack, raid, or scout. If you click the wrong one you will lose the army. There is no undo after they leave.
Carry and crannies
Each unit has a carry cap in the troop table. A village with a Cranny hides a chunk of each resource — 1.5× for Gauls, 80% effective against a Teutonic hero. If your raid comes home with less than the cap, they hit a cranny or an empty warehouse, not a bug.
Oases first
Nature troops sit on oases and respawn on a 48-hour timer. They stay weaker in the first weeks and only reach full size around week 8. Small oases are the safest first fights. Unoccupied oases can also be reinforced on this world, which is not stock Travian — check the tile before you assume it is empty.
Timing on a windowed server
Troops sent at the end of the evening window can land in the morning, or even the next evening, depending on distance and troop speed. That is a feature. Plan the return so you can resend inside a window. See raid loops.