My db tempdb is very big (55 Go), how can i reduce it or erase it and why it's so big ?
to reduce the size try dbcc shrinkfile.
DBCC commands like DBREINDEX and CHECKDB can use quite a bit of tempdb space, espically if they are run on multiple databases at the same time.
Anything that uses a temp tables and probably anything with an order by or group by or subquery will use tempdb.
tempdb will get recreated when you shutdown and restart SQL Server. Don't erase it! - SQL Server will stop working!
You can shrink the files if you wish, but I would say that if tempdb has got to that size on one occasion, the liklihood is that it will get to that size again. By shrinking the file, you are introducing extra overhead on your server by a - shrinking the file (one-off) and b - forcing the file to acquire new disk space when it needs to grow again.