Whom should a muslim consider as a friend and whom should he/she ignore?

Q1. Whom should a muslim consider as a friend and whom should he/she ignore?

Assalaamu Alaikum. I’m 19 years old (male). Throughout the school & high school days I got to know a number of people and used to think them as a friend of mine. But in recent months my longing grew to only be with the people who won’t influence me into sins. Now the sorting out of friends seems to be a problem as like 90% of the people I’m familiar with listen to music and 20% are into alcohol and/or has affair with females. I fear that if I grow closeness with these muslims and hang around with them soon/later their deeds might influence me as mostly they’re into unislamic acts. 

I read in hadith/Quran that a muslim should contact to keep in touch with his close muslim brothers within 3 days and that we should learn to ignore people/places that are involved into worldly/sinful acts. Also that the best friendships are those that are made for the sake of impressing Allah. Now as I think my classmates will have more influence on me than I will have on them with guiding; tell me how to be friendly/contact with them for the sake of Allah when they’re into sins?

I know most of them for 2/3 years now and I think they’ve good hearts as at times when I got into troubles someone always helped me out with constructive advices. So it sounds selfish now if I don’t speak to them for my own good but could take help when I needed right? Do I speak to them again if I have a problem or do I ignore them for good? Kindly explain too on how to treat a cousin in such cases?

Q2. Questions regarding females in a muslim’s family, non-muslims and other people.

1) I know this christian classmate of mine for 6 years now and he tends to think that I am a friend of his. Should I keep in touch with him or should I ignore speaking to him because he will probably have nothing islamic to talk about and might also influence me towards sins? Also won’t I hold the danger of getting punished by Allah in making friendship with a non-muslim

2) Can a muslim be friend of/talk with someone who believes in the singularity of Allah yet dislikes and doesn’t follow the religion because he thinks human’s and satan’s fate shouldn’t have been predefined

3) Who are the females a man can’t get married to?

4) Can a man speak to the females who he can’t marry? 

5) Are we allowed to speak to our parents brother/sisters daughters i.e. our female cousins (the ones who we can’t marry) on phone or online