Ok, so basically what you are doing is adding some flavor and hopefully making it a little more tender.
Marinades are usually a combination of oils, spices, and perhaps a flavored vinegar or citrus fruit.
You can use an Italian salad dressing, a bottled marinade (you will usually find this with the BBQ sauces), or a packaged one you might have to add water, oil, vinegar to (find this usually with the packaged gravies.)
For bottled pre-prepared marinades, I like Earnest J Stubbs and Mojo Carillo (usually Latin foods aisle, and not expensive at all.)
Or, you can experiment and make your own. If you want to do this, start with maybe 1/4 cup olive oil, tablespoon of vinegar (there are lots of different kinds), salt, pepper, and maybe a little ground red pepper – whatever floats your boat. You could even dice up some garlic, perhaps 1/2 teaspoon.
Place the chicken in a big baggie with the marinade and put it in the fridge for at least an hour, longer is better.
But not too long. More than 24 hours is too much, especially if you’ve added lemon juice. Ask me how I know.