Total Annihilation/Tactics and strategies

Wasting resources
In TA there is a maximum amount of energy and metal that can be stored. If you are gaining net metal or energy, and the storage for that resource is full, then the resources you should be getting are simply lost. To avoid wasting resources like this you should spend resources in such a way that your resource storage is never full, or increase your resource storage by building metal and energy storage buildings. If you are about to fill your energy storage you could turn on more metal makers to convert that energy into metal. If you are about to run out of energy, or fill your metal storage, you can turn metal makers off. If you are about to fill both metal and energy storage then you need to build more units and buildings.

Not grouping an attack force
When attacking the enemy, make sure that your forces don't arrive all spread out and so get picked off one by one. Instead, send them all to one spot close to the enemy but out of enemy range, and then send them in. Alternativly, you can have your army guard its slowest unit, then send that one unit on its way. This keeps the group moving together.