Never "choose" to be a worker for Christ, but realized that Christ has chosen you to be his worker. Because he choose us, We can't be allowed to think that we're not qualified in some position, nor can we perform the tasks that Christ has asked us to.
In the past I've joked that I'm not qualified to be a preacher, or a missionary, etc. However if God did call me to one of these tasks, or something else, He would give me the ability to perform the work for His glory.
And before we puff ourselves up thinking we're the greatest preacher that ever lived, or whatever, we need to remember that we are simply sinners saved by grace.