Troubleshooting if your diet is the cause of your lack of energy:
Are you drinking enough water?
Are you eating a variety of foods: different colors, grains, beans, veggies, fruits? as a new vegetarian you should normally hear about and try at least 6 different foods you haven't heard of before.
And to be on the safe side be sure your supplimenting B12 and the like.
Check out:
Vegan Health.org : Home
If none of these address your lack of energy are you sure it's related to your diet? Getting enough sleep?
If that doesn't address the issue see a doctor or natureopath.