A castering nosewheel means you have no control over it - it's just like the wheel on a shopping trolley, no connection to the position of the pedals at all.
Once you are down, if landing, you should use of combination of aerodynamic control using the rudder and differetial braking to keep the aircraft straight. If doing a touch and go just leave the brakes alone and use the rudder to keep the thing straight. This will be easier once you have full power applied giving a better airflow over the rudder.
If you go on to fly something with nosewheel steering, the pedals do steer the nose so you have a lot more control during the ground roll.