rented an appartment with a one year lease my tenent informed me she has to break the lease and leave by the end of April and want

The tenant remains liable for the rent for the balance of the term of the lease.  If the tenant does not continue paying the rent, you can sue for the rent owed.  However, you will need to mitigate (minimize) damages by making reasonable efforts to re-rent the place to another tenant.  You can't allow the place to remain vacant for the remainder of the term of the lease without making reasonable efforts to find another tenant.  If you don't make reasonable efforts to find another tenant, your damages (the amount of compensation you are seeking in your lawsuit against your former tenant) will be reduced accordingly because you failed to mitigate damages.  Once the place is re-rented, your former tenant's obligation to pay rent ends.  However, if the new tenant is paying less rent, the former tenant could remain liable for the difference in rent for the balance of the term of the former tenant's lease.  Again, you need to mitigate damages and would need a valid reason such as market conditions for charging the new tenant less rent than the former tenant.

Reasonable efforts to find another tenant would be determined by what other landlords in the area are doing to attract new tenants.  For example, posting a sign on the property advertising the vacancy, advertising the rental in the newspaper or online, or in a local rental guide, etc.