How do I masterbate ?

Get one of those door-stop* things... you know what I'm talking about.
They look like a wedge and people use them to prop doors open, in your case... you'll use it to prevent the door from being opened.
(it works)

first you will need a Nigerian man and a contraption known as a penis enlarger... soon you'll require a cylindrical item, such as a tin can and jiggle it around. That Nigerian man is there to watch. Try not to cut it, as all the stuff will spill out like a ketchup packet. Get a bucket of warm milk and pour it into the can. Goat milk recommended. Rinse and repeat for a week. Don't feed the Nigerian. Then clean up everything, takea shower, go to bed, wake p and masturbate. That's the simplest answer.

No lock on the bathroom door? They come into your room in the middle of the night
Why?

Wait until everyone is asleep or wake up early before everyone else. 

Do it once they've gone to sleep or teach them to knock before coming in. If you have a bath tub you could also have a bath and play music

Put a lock on your door. You are entitled to your privacy.

lock the door and masturbate away

Have a family meeting and clearly state that you intend to masturbate several times a day and you expect privacy.

Replace the doorknob on your room with one that has a lock. Replacing the knob is not difficult. All you need a is a screw driver. A replacement with lock can be purchased at a local hardware store, online, or at walmart.

There are videos on youtube demonstrating how to do it.

Ask your parents for a lock for your door telling them you can relax with everyone just come in and out of your room 

Or do it when everyone is asleep or just stick to the bathroom 

So you're ok with letting all the oh so fun and understanding people on y/a know your masturbation schedule but not ok with locking a door and seem to be hung up on master? And your family walks in on you? What do they do, have a daily gather to catch you pulling your pud? Are you Beavis or Butthead? We should really consider teaching common sense in schools. 
Then again you could be a skevey ped or a commie Troll because even the most juvenile with any sense at all should be able to solve that after at least the first time it happened. 

Do it at night.

Get a dog or a cat to be in the room with you.  The animal will alert you when someone is approaching the door.

Find some way to lock or block your door.

When my wife and I were on vacation this summer the beach house we rented didn't have a lock on the door, so to keep our kids out when we wanted to f**k, we would slide a dresser over in front of the door and 'voila'.

 You need to focus on something else..... distract yourself. Then use a penis care product when you've showered and are ready to either face or say goodbye to the day. You'll be astounded at the amazing benefits, especially increased blood flow, repair to nerve damage- and even a fresh scent!

do it in the shower