I would like to know the purpose of Tribulation and who will be going through it. Where will the unbelievers who are already dead? Will they just be in their grave during the Tribulation?
Thank you for you attention.
Before I provide any one answer, I want to point you towards some resources. Tribulationism (as well as most of Eschatology) is highly debated. Equally spirit filled theologians will argue whether the tribulation will happen before, during or after a literal rapture, and will even argue whether or not the tribulation has already occurred. For a brief overview, you can't go wrong with this Theopedia.com article
or a broader, perhaps even more academic overview on this Wikipedia.com article
While not directly stated in the Bible I believe there are a few great starting points for possible reasons for the tribulation. My favorite is that the Great Tribulation will serve as both a catalyst and a line in the sand for the people of the world. Jesus warns that in the last days there will be many who will have a form of godliness but deny His power (2 Timothy 3:1-9).
During the Tribulation there will be clear sides. This will not only be a forerunner to the Biblical battle of Armageddon, but will also serve as a catalyst for evangelism.
As for who will be going through the Great Tribulation, I am very comfortable saying that those already deceased will continue to sleep during this period. When the rapture occurs the faithful will be resurrected to life while the lost will be resurrected to death. Beyond that (whether the resurrection will occur before, during or after the tribulation) I cannot, with academic integrity, express one position as absolutely true. My view holds that living believers will experience the Great Tribulation.
This may muddy the waters some, but the Great Tribulation has been and always will be a hotly contested aspect of theology. Preterists believe the Tribulation occurred around 70 AD and included the destruction of the temple. Some even believe the return of Christ occurred around this same time. Meanwhile, many academics argue that the Church will be completely spared from an upcoming tribulation. The best I can do is applaud the curiosity and willingness to learn more about eschatology and encourage you to continue reading, asking and studying the Scriptures.
I hope this at least gives you a great starting point. As always, I welcome any feedback!