You do not have to make up all missed prayers but you have to repent your sins and increase of making supererogatory prayers which may make up some of the salahs you missed in the past. In order to gain forgiveness, scholars have stated that the following conditions must be met:
1. Giving up of the sin itself.
2. Remorse over what has been committed.
3. A resolve not to repeat it

4. Compensating those who have been wronged or obtaining their forgiveness.