I am a recent college graduate, and I currently have a secular job with a high salary. I don't know, but I always feel sad whenever I'm going to work, thinking that this is not the world I wanted to be in, this is not the job I wanted to do. In short I'm not happy because I have the burning desire to be a full-time missionary. I have a desire to study theology and be a full time servant of God. That would make me happy. But i don't know what to do because I know I need the job, and I still don't know how I'm going to start with my desire. I hope you could help me.
First, the idea that you can only serve God completely by being a preacher is false. "Bondservants, be obedient to those who are your masters according to the flesh, with fear and trembling, in sincerity of heart, as to Christ; not with eyeservice, as men-pleasers, but as bondservants of Christ, doing the will of God from the heart, with goodwill doing service, as to the Lord, and not to men, knowing that whatever good anyone does, he will receive the same from the Lord, whether he is a slave or free" (Ephesians 6:5-8). Even in a job you are doing the work of the Lord.
However, nothing says you cannot begin preparing for preaching while you work. Start learning all you can from other preachers. "And the things that you have heard from me among many witnesses, commit these to faithful men who will be able to teach others also" (II Timothy 2:2). Study your Bible. "Be diligent to present yourself approved to God, a worker who does not need to be ashamed, rightly dividing the word of truth" (II Timothy 2:15). Start teaching and preaching in your local congregation. The idea that you have to attend some man-made organization and get a degree in a man-made topic is false.
I'll use myself as an example. I decided to preach at the age of 16. I chose a career field of computer programming because I knew I could get a job anywhere in the country. As I studied for my career, I would preach wherever I could. I taught classes. I kept careful notes of the lessons I heard. When it came time to begin my secular work, I purposely chose regions where the church was small, weak, and could not afford a full-time preacher. I would preach in those congregations and work to spread the gospel in that area. Eventually there came a time that my secular career began to interfere with my preaching. I then switched. I found a congregation where I could preach full-time and I work in my secular field part-time to supplement my income. I know many preachers who have done the same thing.