I'm going to change the wallpaper and change mythe room's door.,

B because there are a lot of stain on the wall.

At first I couldn't decide what color I should I choose for wallpaper?.

When I was young, I use to like redthe color. red.

So I bought a red shirt and red sneakers.

When I was 30 years old, I used to like blue so I bought blue jacket and blue car.

And now, I like pink color, So I decide, So I decided to put up pink wallpaper.

I'm going to change the wallpaper and change my room'sin my room and on the door.

I assume you are not changing the door itself?

Because there are a lot of stains on the wall.

At first, I couldn't decide what color should I chooon which color to use for the wallpaper?.

Because athe constructor has tons ofso many sample colors and patterns options.

Between“sample” and “option”you can just choose one to keep. If you keep “option” then you can have a sentence like “because the constructor has so many color and pattern options”

When I was young, I use to like red(the color) red.

Words in brackets are optional to include.

So I bought a red shirt and some (or a pair of) red sneakers.

When I was 30 years old, I use to liked blue, so I bought a blue jacket and a blue car.

And now, I like pink color, So I decide. So I decided to put up pink wallpaper.

I'm going to change the wallpaper andbecause there are a lot of stains on the wall and also change my room's door.

Because there are a lot of stain on the wall.

I combined this sentence to the sentence above

At first I couldn't decide what color I should I choose for the wallpaper? because the constructor had tons of sample colors and patterns to choose from

More natural: At first I couldn't decide on a color for the wallpaper because there were so many options

Because a constructor has tons of sample color and patterns options.

Usually we try not to start a sentence with "because" so it is better to combine sentences

When I was young, I use to like redthe color red.

So I bought a red shirt (or "red shirts" if pleural) and red sneakers.

When I was 30 years old, I used to like the color blue so I bought a blue jacket and a blue car.

And now, I like pinkthe color pink, Sso I decide I decided to put up pink wallpaper.

I'm going to change the wallpaper and change my room's door.the door to my room,

Your sentence is correct, but I think this sounds a bit more natural.

Bbecause there are a lot of stains on the wall.

I connected the two sentences. A clause starting with "because" must be connected to an independent clause.

At first I couldn't decide what color I should I choose for the wallpaper?,

Here you used an indirect question construction (for example, "I did not know who...." or "I finally decided which..."), so there's no switch in word order. Compare "Should I choose red for the wallpaper?" (here "I" and "should" are switched because this is a direct question) with "I couldn't decide what color I should choose for the wallpaper" (here there's no switching because it's an indirect question).

Bbecause athe constructor hasd tons of sample colors and patterns options.

When I was young, I use to like redthe color red.

So I bought a red shirt and red sneakers.

Here you need to use the indefinite article "a" before a singular object.

When I was 30 years old, I use to like the color blue so I bought a blue jacket and a blue car.

And now, I like pinkthe color, So I decide pink, so I decided to put up pink wallpaper.

I'm going to change the wallpaper and change my room's door.

Bbecause there are a lot of stains on the wall.

Connect this with your previous line, because it can't stand alone.

If you mean more than one wall, it should be plural (walls)

At first, I couldn't decide what color I should I choose for wallpaper?

Although you are describing wondering something yourself, this isn't phrased as a question, so no question mark.

Bbecause a construchome improvement stores hasve tons of sample colors and patterns options.

This is a sentence fragment, it should be attached to your previous line.

I'm not sure if you have hired a professional or if you are purchasing the paint/wallpaper/door yourself. If it's the latter, the phrase I wrote is the more natural way to say this. But if you hired a person you are working with, you could instead say "because the home renovator I'm working with has tons of..."

When I was young, I used to like redthe color red.

So I bought a red shirt and red sneakers.

When I was 30 years old, I used to like blue so I bought a blue jacket and a blue car.

And now, I like pinkthe color, So I decide pink, so I decided to put up pink wallpaper.