I have my transaction log shipping every 2 days, plus daily backup of the db. My hard disk is 500 GB, but the database size is already reaching 490 GB. I'm afraid it'll run out of diskspace soon.
Taking transaction log backups, as are required for log shipping, internally clears portions of the transaction log, allowing the physical space on disk to be reused.
Assuming it's the size of the log file that's the problem, the frequency of the log backups should be increased. This will mean less physical disk space is required for ongoing operations; the physical file could then be shrunk to reclaim the unused space, probably so it can be used by data file growth.
If you still only want to do log restores on the secondary once every 2 days, that's fine -- the frequency of the backup/copy/restore jobs can be set independently.
Note that this only applies to log files.
If it's the data portion of the database that is growing and the amount of log being generated isn't necessarily increasing (and is a small part of the total space used), you will need to either (a) provision more storage, or (b) implement some kind of archiving/data deletion process, as there is no built-in mechanism that will clear out data.