How do you make letters with the two dots on it on a computer keyboard such as an a with the two dots over it or a line?

If this is for Microsoft Word or publisher you can go to insert and then select symbol, and choose which character you want (like the a with an accent or o with two dots above) If this is not for Word or publisher, then you can copy it from Word or publisher. Or go to the Windows start menu and select the character map.