There is no set strategy for defending yourself during the early part of a game, strategies vary greatly between maps and sides, there are just too many variables. However, a basic, flexible concept can be applied as a core plan.
Heres a quick break down based on faction
Americans- Early on, riflemen are your best bet. Get them sticky grenades and then BARs. This will give you good enough AT (If you have munitions to support it) Riflemen can hold their own against almost anything except artillery, mortars, or heavy machine gun fire. They are also cheap to produce and reinforce, have a low upkeep cost, and can be reformed from medic stations. This versatility makes them one of the best units for a flexible defense.
British- British are a little tough to defend for an early rush, they really only have two units, infantry sections and bren carriers. Fortunately, they have some of the strongest infantry squads, which can easily beat another squad one v. one. If you need to be moving around a lot, your best bet would be to get the recon upgrade. While this makes your infantry a little less offensively effective, it gives you the sniper ability and a faster squad. The sniper ability is great for taking out weapon crews since the squad quickly mop up the other two of the three crew members. Otherwise, use the bren upgrade for some fire support. Bren carriers are good for quick response to attacks, but are expensive for their low health. Used to support infantry sections, bren carriers can really help out.
Wermacht- Wermacht are one of the most flexible early game factions there is. They have quick access to strong infantry. With just an upgrade to level 1 veterancy and mp44's, volks are one of the most dangerous threats in the early game. The level 1 veterancy makes them more accurate and gives all infantry the ability to slowly regenerate health when not in combat. With MP44s, you just literally run past squads and they get mowed down. When armor attacks, you can use muntions for panzerfausts, which do a ton of damage.
Panzer Elite- They have panzer grenadiers, which are amazing. Use them for EVERYTHING. Get an infantry halftrack to support them,. allowing for mobile reinforcement, and you are set.