The first step to take when you notice that the home you are renting needs repairs is to let your landlord know what needs to be fixed. As with most things, it's usually best to start by asking nicely! While demanding repairs in writing is a good idea (and is sometimes legally required), it may be better to give your landlord a phone call, text message, or ask in person first. Sometimes a formal letter can make people fearful or defensive and that may not be the first thing you want to do.
But, sometimes it makes sense to start with a letter.
For example, if the problems are very serious, if you're concerned about your health or safety, or if you're pretty sure that your landlord isn't going to respond unless you let them know you are serious.
You have to use your best judgment to make this decision. But remember that having a good working relationship with your landlord, if possible, can make things go more smoothly.