3 stats are going to decide the outcome of your efforts at the end of the game: Total Military Strength, Galactic Readiness Rating and Effective Military Strength.
Total Military Strength: The accumulation of all the war assets and support you gathered.
Galactic Readiness Rating: Base is 50% (higher if you play multiplayer)
Effective Military Strength: It is this number that will decide the outcome. Its value equals the percentage indicated in GRR of your Total Military Strength.
Basically, if you don't play multiplayer, all your assets and support counts for half its value. You have to work twice as hard to get to the best possible outcome (played all the ME games yourself with all the DLCs, including ME3 and you need to explore and recover all assets in the majority of the star clusters available to you.The aim is 5000 Effective Military Strength, although the "best" ending is available at 4000 if you are quick to react at the end.