My three best friends are all agnostics or atheists or something like that. They don´t seem to care about my faith. One of them swears very often, and we tell many kind of perverted jokes or innuendo-jokes and things like that when I see them. I don't know if they are very good influence for me. But they care for their friends, me too, are friendly, don't bully anyone and so on. Actually, one of them has been bullied. Of course, I care for them too, and love them as friends.
I read an answer to a question on this site, and you said something like "you should be friendly with non-Christians, but not friends" because they are bad influence. But if I would leave them, I would have no friends. One of my friends, who has been bullied, has been depressed, has been cutting and thinking of suicide and things like that, but now she seems to be doing a bit better. But if I would leave her, I don't know, what she would do. It was this year when she was really depressed, and there are times when she still has suicidal thoughts.
What should I do? Should I talk to them about this or just let them believe in what they want to believe in?
Notice that they are influencing you. You tell sexual jokes, but Paul said, "But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks. For this you know, that no fornicator, unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and God. Let no one deceive you with empty words, for because of these things the wrath of God comes upon the sons of disobedience. Therefore do not be partakers with them" (Ephesians 5:3-7).
The problem when you have close worldly friends is that you start compromising your standards to be accepted by them. I'm not saying that you must leave them. "I wrote to you in my epistle not to keep company with sexually immoral people. Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world" (I Corinthians 5:9-10). But I am saying that you can't let your guard against sin down when you are around them.
What a Christian is required to do is spread their faith to others by both talking to others about God's teachings and by showing others what a real Christian's life should be like. "Let your light so shine before men, that they may see your good works and glorify your Father in heaven" (Matthew 5:16). Some people won't like it because it is different. "For we have spent enough of our past lifetime in doing the will of the Gentiles -- when we walked in lewdness, lusts, drunkenness, revelries, drinking parties, and abominable idolatries. In regard to these, they think it strange that you do not run with them in the same flood of dissipation, speaking evil of you" (I Peter 4:3-4).
What you should do is add more friends, especially Christian friends since you seem to be lacking in them.