Both "Where can I weight my groceries?" and "Where can I weigh my groceries?" are similar phrases, but the correct one is:
"Where can I weigh my groceries?"
In this context, "weigh" is the correct verb to use when referring to measuring the weight of your groceries on a scale. "Weight" is a noun that refers to the measurement of how heavy something is, while "weigh" is the verb that means to determine the weight of something.