How do i make a letter like the i in this word (Sïn)?

Answer 1

Capital is ALT+0207
Lower case is ALT+0239

Hold ALT while pressing those numbers on the 10-key pad, then release ALT.

Answer 2

The symbol is called an Ümlaut in German.
To insert it into your Microsoft Word document, do the following:

Place the cursor where you want the letter to appear
On the top menu, click Insert, Symbol.
Select the ï from the symbols, then click on Close.

That's it. :)

Answer 3

hold the ALT key on your keyboard while pressing 0239 on your numberpad.