First up, note down your phones SW/firmware version (find by pressing *#0000# at standby screen, not applicable if the phone is dead), product code (on the sticker under battery in the form of 0XXXXXX) and type (look at the sticker under the battery, it is in the form of RM-xxx).
WARNING: Phone flashing is a double-edged sword; it may be useful and dangerous at the same time. It may brick/damage your phone for good (if not done carefully), so please DO NOT flash your device if you don’t know what the hell flashing a phone means