Who can become a labourer in God's field?
The answer to 'Who can be a labourer in God's field?' is anyone. It is not up to us to choose whether they are right for the job (in other words, whether they have a vocation), that task will be taken up by the church as a whole. Each denomination will have their own ways of testing such a vocation, and it can take years to test and train such ministers.
However, Jesus wanted his disciples to pray that labourers would be forthcoming in the harvest field. Our task is to keep on praying that people with specific skills and those willing to learn and grow as Christians will be found. As a starter, therefore, you could make a list of those Christians you know whom God might call to such positions. Start small: