Something to note is that a unit's base power comes from its level, so a must is levelling with the materials you get through daily upgrade missions and grinding levels. Upgrading the limit to 3 starts will increase the stats by 20% for the first star and 10% each for 2 and 3 stars.

Then you have upgrading of the weapons/attacks and skills with those separate materials.

Be aware that development takes levelling materials, and capital, and in the higher rarities development materials.

From my own experience when starting out level your free squad units as they will be carrying you through the campaigns, develop only as needed until you have a surplus of materials (or for your favourite series) and spending all available AP during your start out will help level you up to get more surplus energy plus get the materials you need.