Below are different vulnerable SQLi targets. Think of those pages as an online store. Try to obtain the 'date of birth' and 'products ordered' from any customer. Pick your desired target and take into consideration the different levels of security that may apply for each sector.
Online Store for Electronics Online Store for Model Trains Online Store for Flowers
The method of SQLi is based on escaping the matrix... more or less. Commonly you will be using search forms and input fields.
Example:
Input "Banana" into a search box. The site will instantly answer with "Here are possible Banana results". Now, if you instead input "Banana´ AND sleep(100)" it will wait 100 milliseconds before answering. You can now inject different characters and sql statements and see what results you get.