"What if I don't know any unbelievers?"
I'd like to address this question whether you've been a believer in Jesus for many years, or you are a new Christian. Here are some brief thoughts.
1. Yes, you do. You may feel like the only people you know are Christians--and that may be true for the most part--but that's likely not the case if you really take a look around. Forgetting members of our own family we are unsure of in their relationship with Christ, what about people we work with, go to school with, or see frequently in the neighborhood? Have you taken time to hear their story--ask them questions--really get to know them? Do you know their religious beliefs? Think about all the people you interact with in a given week. Now read Acts 17:26 and imagine if God really has "determined...the boundaries of their dwelling place" to interact with you. Begin writing their names down. Start by praying for them.
2. Don't Over Think It. Many times we fail to see the people that God has providentially put in front of us because we are caught up in looking around for someone else. We have an image of a "kind" of person that we should reach out to and totally miss the weird guy at work, the same lady at the pharmacy we regularly talk to, or the kid that asks the strange questions in Sunday School. Those are people to relationally invest in. Start there. Get to know them. Hear their story. Share yours.
3. Repent. There will always be unique challenges to engaging people with the gospel, but most of the time we have something in our hearts that we need to turn from. Sometimes this is laziness to engage getting to know someone. We think it will take too much of our time and will drain us. Sometimes it's a fear of what it will cost us to "out" ourselves as believers in Jesus at work. And all of this stems from an unbelief in the power of the gospel to save--and being overly focused on retaining our comfort or reputation. If Jesus embraced being misunderstood by people in his love for them--we have to be as well (Luke 7:34; Matt 10;25).
4. Start Simple. The only way to fix the problem of not knowing any unbelievers is to intentionally get to know unbelievers. I know. It's really profound. But sometimes we think we can share the gospel without getting messy relationally. Sometimes evangelistic methods even encourage this. You can be trained to busily pass out gospel tracts but not be encouraged to get to know the person you're entering into one-way conversations with. You are encouraged to sign up for the mission trip--but not encouraged to walk across the street to the unglamorous neighbor and invite him to watch the game. If you want to share the gospel--you've got to appropriately open up your life to people (1 Thess. 2:8).