You can completely miss your ovulation, for years, if you are only having sex once a week. you need to try for at least twice a week. That shouldn't be such a chore for a young married couple, and since you don't want to spend any money, more sex is your cheapest option.
You should be taking folic acid if you are trying to get pregnant, but do not waste your money on any vitamin, mineral, or herbal supplements that claim to "help you conceive." They don't.