Sleeping the same amount every night is important, but it's just as important to go to sleep at the same time and wake up at the same time every night. For example, if you sleep 10pm-5am one night, then midnight-7am the next, and 3-10am on weekends, you'll probably still feel tired even though you're getting about 7 hours of sleep every night. Our bodies work better on a regular schedule; figure out what time of night you typically get sleepy as well as the number of hours you need to feel rested, then develop a schedule from that.
Without any other information, that is the best advice I can give...make sure you have talked to your doctor about diet, depression, thyroid conditions, or sleep disorders. Sleep apnea, for instance, will leave you feeling tired even if you get "a good night's sleep" because your body isn't getting enough oxygen.