Some things you can get on the packaging the ingredient came in.
For example if you're baking a cake take the nutrients of the flour, butter, eggs and sugar off their packaging and calculate to the amount you are using, add together and you have the nutrient value for your cake.
Some eggs won't have the nutrients on the carton so google the egg nutrients; for example:
*free range medium chicken egg or * large organic duck egg
Do the same for things like vegetables, salads and fruits. Be sure to include where they came from ie market, supermarket and what country.
Meat too if packed should have the nutrient values on it. If not google the meat: for example:
*5 oz of fillet steak or 3 oz of cooked chicken fillet
Some cook books carry nutrient values of ingredients in their back pages. If you're going to write a lot of recipes it might be worth investing in a really good book of different food nutrition values.
And then write lots of hubs.
Hope this works for you :)