Firstly, congratulations!

Now to your questions:

Vitamins: If you aren’t taking it already, start taking folic acid today, at least till you are 12 weeks pregnant. If you aren’t already taking it, then bear in mind for the future, that if you decide to have another child you should start taking it from the day you start trying to get pregnant. You want 400mcg (micro grams) per day.

If you get particularly tired, you might need to take iron.

Vitamin D is important (10mcg per day). If you don’t get a lot of sunlight or if you have particularly dark skin, then consider taking a supplement.

Vitamin C and Calcium are important too, but you just need to eat a balancedcan easily get enough from your diet.

Vitamin A (retinol), should be avoided. Too much can be harmful, so don’t take a supplement that includes it.

If you are vegan or have a gluten-free diet, then speak to a doctor or dietician (not a “nutritionist”), to be sure you are getting what you need in your diet and otherwise take appropriate additional supplements.

Telling friends: I would tell my best friend soon. A lot of people wait till 12 weeks in case there are issues (there are issues in the first 12 weeks a lot more often than you probably imaging), but imho if there are issues, you might want your best friend to b me there for you.

Sleeping: Yes. A baby can sleep anywhere that is safe. A walk-in-closet is just a small room. However if there’s no windows be conscious of temperature.

Dentist: You can and should go; just be sure to tell them you are pregnant. In the UK, you get free dental treatment while pregnant.

Tests: False positives are incredibly rare. False negatives are more likely, particularly with out-of-date tests, but even an out of date test that says you are pregnant is almost certainly correct.