I want my emails to have capital letters to start sentences and for the letter "I"

When I am composing emails, the first letter of a sentence is not capitalized. ...

If you are using the editor at https://connect.xfinity.com you'll have to type capitals manually. The editor does not have that feature. 

... I want the letter "I" to be capitalized.

If you are using a sans-serif font like Arial, you my need to switch to some other font, perhaps a serif font like Times New Roman.