For most cases you would use "få vänner" however if there is an emphasis on the action you use "skaffa vänner".
"You will make new friends" - "Du kommer få nya vänner."
"You need to make new friends." - "Du måste skaffa nya vänner."
I know you miss out the att in spoken speech but do you actually miss out the att in written too? (Regarding kommer att)
Yes, you may do. (But it is not wrong to have att after kommer). I think that in formal text it is still more common to have the att than to leave it out.
There are a few articles in Swedish about it if you want to try to read them: http://www.iklartext.se/kan-man-skriva-kommer-utan-att/ http://sverigesradio.se/sida/artikel.aspx?programid=411&artikel=4589380
This verb is only for Internet stuff? If I want to add honey in a cake's recipe? It works?
If you would be using this sentence for that, you would basicly be putting your friends in a cake.
It's not the same thing. You can add new friends to a list or on social media, but when you make friends, that means you become friends with someone. That would be Jag får (or skaffar) nya vänner in Swedish.
It took me two hours (a month ago) to translate this book about making friends: https://www.google.com/url?q=http://www.childrenslibrary.org/icdl/BookPreview?bookid%3Dwilenok_00940056%26route%3Dsimple_0_0_0_English_315%26lang%3DEnglish%26msg%3D%26ilang%3DEnglish&sa=D&source=hangouts&ust=1550675711426000&usg=AFQjCNElyL7MpFCbrx4BGfcuM4sKvMUxPw