First and foremost, you should be consistent with the measurement that you use. However, for protein sources, you do need to measure in grams or ounces for the most accurate information.
Overall, using a food scale to measure in grams or ounces will be the most accurate measurement. If you are just starting out and getting into the habit of tracking & measuring your food, starting out with cups as a measurement is completely fine!
If you do not have a food scale, you also don’t need to wait to start tracking and measuring your food until it arrives. Start out with using cups as a measurement to track and then transition to using the food scale.
If you need to purchase a food scale, they are relatively inexpensive, and here is an option you could purchase or use to find others.