Dimi,
which country are you wanting to stud in?
Regardless, you might want to consider the reasons why you didn't do so well in your undergraduate course. It may be a harsh truth that you're simply not cut out for studying and therefore it wouldn't be appropriate for you to waste further time and money doing a masters.
On the other hand, if the reasons were due to external factors, and you honestly believe you can do well on a masters course (and enjoy it!), then go for it. There will be nothing worse than starting on a masters and failing it.
Good luck.
T