Combining the information from the other two answers and adding some further details:
The methods available for adding characters beyond the English 26 letters depend on your operating system. Macs and i-things behave differently from Windows, and I have no idea what Android devices and Chromebooks do.
I am not sufficiently familiar with Macs to offer instructions for them. On iPads, if you "hold down" a key you'll get a popup with the available modifications for that character to choose from. (I have no idea what you can do if the diacritic you need isn't in the popup.)
One method that will work in some fashion in any system is to copy-and-paste the character from somewhere else. The exact method by which one copies and pastes depends on the OS; in Windows, you can select the text, hold down the Ctrl key and type C (for Copy), then put the cursor where you want the letter, hold down Ctrl and type V (for the copyeditor's "insert" symbol). Here are a few webpages for the "somewhere else":
In Windows, for occasional use of the more common modified characters, the easiest input method is alt-num codes. For these, you need to use the numeric keypad; the codes do not work with the numerals on the top row of your keyboard. The second and third links above give some of the codes and instructions. The basic idea is that you hold down the Alt key and type a three- or four-digit number on the keypad, then release the Alt key. For example, to type ß (the German ess-zet), the code is 0223, and for é (e-acute), it's 0233. (Yes, I often get the one when I wanted the other.)
Still in Windows, if you need to type extended text in a language that uses diacritics, and/or you frequently need a character that does not have an alt-num code (such as ő and ű, with the double-acute accent that's almost unique to Hungarian), you'll want to install an alternate keyboard layout/language. The specifics depend on the version of Windows, but you start with the Control Panel.
(Note that switching keyboards/languages will affect the alt-num codes, which is a detail ignored by most of the websites that give lists of codes. As an example, if my keyboard is set to English, Alt-0232 gives è [e-grave]; if it's set to Hungarian, the same code gives č [c-haček].)
Related to the copy-and-paste method is the Character Map utility of Windows, which basically offers a source of accented characters even if you are not online.