Restaurants: Usually not a big problem. You can ask to replace potatoes or fries with a salad or low-carb vegetables. Ask for extra butter.
Fast food: At hamburger chains the hamburgers (without the bun) are usually the least bad option. Avoid soft drinks and fries, obviously. Drink water or unsweetened iced tea. Pizza toppings are usually OK. Simply remove from the crust and enjoy.
Nuts or cheese are good “emergency food” when there are no other low-carb options to be found. Remember to ask they leave the crackers out, possibly substituting walnuts or almonds, you may even get more cheese!
If you eat strictly everyday it’s less of a problem to make a few exceptions when you are invited out. If you’re not sure what will be served you can eat something at home before you leave.