If you must reconcile with a girl, you must acknowledge that you have offended her, apologize for your wrong doing and ensure it never repeat itself. You really don't need to buy her a gift just because you want to reconcile with her because it is possible to buy her a gift to show that you feel remorse for your wrong doings but you still repeat same mistake. The most important thing is sincerity of purpose and trust else it won't work. Integrity plays a major role here. Learn to keep to your words after making promises not to hurt her.