First day on my Keto Diet- will I lost my muscle by not taking in as much protein?

I'm not convinced you should limit protein when eating a very low carb ketogenic diet. How much protein you should consume is going to depend on your current lean mass, what type of exercise you are doing, and how much of that exercise you perform.

However, you should require less protein to sustain or build lean mass than a non-keto dieter because the ketogenic state is leucine-sparing. In fact, there is a study where 3 different diets were compared in obese men - all with 1,800 calories and 115g protein daily, but with different carb and fat amounts. The lowest of the 3 diets (30g carbs daily) resulted in the most fat loss and the best sustained lean tissue.

Reference to the study mentioned: Young CM, Scanlon SS, Im HS, Lutwak L: Effect of body composition and other parameters in obese young men of carbohydrate level of reduction diet. Am J clin Nutr 1971, 24(3):290-296.

No need to lower protein on a ketogenic diet. The key thing is low carbs. The old Atkins and Protein Power are both higher protein, low carb or ketogenic diets.