Please excuse me if this has been discussed before here but I have had problems with this myself and thought I would mention it for any one starting out.
On my Windows 10 laptop I added Ελληνικά from the language pack in Windows and can use the keyboard quite easily by pressing the semi colon ; on my English laptop keyboard before typing in the Greek vowel which then places the τόνος correctly. However...……….I have just bought a Windows tablet and this did not work because the keyboard is virtual and the ; on this is of course the Greek ? mark!!
I have now realised that to get the τόνος all I have to do is when typing the vowel that needs the τόνος over it is to hold the letter down with my finger for an extra second and this makes a separate box appear above the keyboard which shows the vowel with the τόνος. Touching on that then places the vowel in the word.
I realise that most of you will know this but it took me ages to work out how to do this (I am elderly) and it might help someone!