You really should take prenatals daily throughout the entire pregnancy. Also you should try to eat a heathly balanced diet and lots of water, even when you don't feel like eating... Other then that there is really not much you can do.
Prenatals are especially important during first trimester so try to take them every day. Try before bed with a light snack to reduce possible nausea.
You can get prenatals from the pharmacy. My doc said there was no reason to get prescription prenatals, that otc were just as good. She recommended nature made prenatal capsules with dha. The dha is important. Often Walgreen's has nature made b1g1 which is nice. You'll need everything in the multivitamin more now than normal cause baby is using up your vitamins.