Add more space - if possible

Purge data - to release space from inside the data files, will need to be done in small chunks due to it needing to log the purge for ACIDity reasons

Perform index and statistic maintenance - to ensure everything is optimal for the queries being performed

What is the recovery model of the database

SELECT recovery_model_desc FROM sys.databases WHERE name = 'databasename'

What is the backup strategy for the database? How often are full, differential, transaction log backups done?