G'day Marcus,
You need to set your CG with the fuel tank EMPTY, move batteries, or add weight, whatever, to have it on the advised point, usually on the spar with a trainer wing.
When flying, with a full tank, it will be slightly nose heavy, & so be a little less efficient on the elevators & as the fuel burns off, the CG will come back closer to the advised point.
After a few flights, your instructor may advise you to move the CG back to find the sweet spot, but that will determined after flight testing.