Index File growing big

Hi all

I have a DB with 1 data file, 1 log file and 1 index file.
data file is 3 GB but index file is 12 GB.
Index file is growing big day by day.
This cause performance of DB down.
What should I do to prevent index file become bigger and size of index file smaller?

Thanks in advanced
Thi Nguyenhelp me.

Thi Nguyen|||Do you have ANY index maintenance tasks implemented?|||Hi rdjabarov
What do you mean "ANY index maintenance tasks" ?
I have many indexes in DB.

Thi Nguyen|||do u mean transaction log file as index file...?|||I meant that index file growing big, not transaction log.
Thanks