Basically, you'll be eating lots of fats like butter, avocado, olive oil, nuts, etc., along with protein and above-ground vegetables, mostly green, leafy ones. You'll be avoiding root vegetables, corn, peas, beans, lentils, anything made with any kind of starch (any type of grain or bean flour) and anything with sugar.
One important thing is to read the ingredient labels on everything. Labels are tricky. Some will say no sugar added, others will report 0 carbs, but they misrepresent this. The FDA allows them to report 0 carbs if A SERVING has less than 1/2 a carb or something like that. Read the serving size, sometimes that's a TEASPOON. Read the ingredients list. It's best to avoid as many ready-made foods as possible. You will miss chips, but you can eat plain pork rinds.
There are lots of Keto videos on YouTube, and I didn't include the longer ones. Here are a few short videos to give you a start: