To be able to choose your side (Axe or Allies), class (Soldat, Medic, etc) and weapons, you have to go to the Limbo menu (L key).
Beginners often choose soldier with the submachine gun by default (MP40/Thompson). However, if you want to gain skills as a soldier you have to choose a heavy weapon: flamethrower, bazooka, mortar or MG (heavy machine gun).
To know the maps and the objectives to be carried out by your team, it is interesting to choose engineer. Engineers are a very important class in the game because they are the ones who achieve the objectives most of the time: build / destroy, repair a vehicle. It is a good way to learn the game!