When you raise the mainsail, you should first release all the devices that could be holding the sail or boom down. The mainsheet should be eased a bit, as well as the mainsail downhaul, the boom vang, the cunningham, and also any stops that might be in the mainsail track above the boom. When there are no devices that are holding the sail or boom down, then raise the mainsail as high as it will go, and cleat the halyard. Then use whatever sail tensioning devices that you have (The C25 should have a mainsail downhaul.) to tension the luff of the mainsail appropriately for the strength of the ambient wind.
The height of the boom will depend on whether your boat is a standard rig or tall rig C25, and whether the sail is a standard sized sail for the boat. (Some owners of tall rig boats put smaller mainsails on them, to raise the boom higher.)