Ear training is one of the keys to great singing. Unfortunately, most of us do not ‘hear’ ourselves correctly and thus have problems with intonation.
If you’re having serious tuning issues, spending a half hour once a week with a teacher will probably not get you up to speed fast enough.
Here’s a great little program to help you visually see what you’re singing and ‘fix’ your ear. Unfortunately, it’s only available for the mac.