When we read a picture of the MC is built into our heads. Now the trick in writing is getting yourself to understand your character and sometimes deciding how he looks can help because it makes you ask fundamental questions about both your character and story.
In what time frame does your story take place.
You have to decide this before you can truly decide how your character look.
In what social class is your MC.
Different social classes has different things that is expected to be proper. The rich lady can afford to buy gem covered dresses while the street child would be lucky if his brown shirt fits him properly.
What your MC occupation is.
The smith will wear a leather apron while the rich merchant might carry with him a gold cane.
Peoples clothes must not get in the way of their job and is often suited to help with it.
Who lives with your MC.
Your MC might be a poor boy who lives with his grandmother. depending on what kind of Character she is she might try her best to keep his clothes in good condition even though they are hand me downs.
Things like this can add a bit of deepness to your MC which piled with all the other small details become important.