First fruit is the first fruit of labour. In agriculture, it is the first agricultural produce of a season usually given to God as a sacrifice.
If you are a worker, it will be the first salary gotten from your place of work or the first salary of the year.
As for where it should be paid, it should be where you are fed spiritually.
For instance, if you attend a church that meets your spiritual need, think it should be paid there. If it's also someone(spiritual person) who meets your spiritual need, like your spiritual mentor, it can be paid to him.
First fruit is a part of your first produce of labour dedicated to God as a sacrifice. Unlike tithes that has a named portion of it being 10% to be given, first fruits is not specific. So you give according to your likeness and willingness.
As for where you are supposed to pay it, in the times of old, it was given to the priest of the temple who will select the best parts of it and offer it as a burnt offering unto the Lord and the rest was reserved for the priest and levites who worked in the house of the Lord. Today, we are supposed to offer our first fruits to the sheperd of the church who is the Pastor. After which, he is to pray for you and bless you afterwards.
These first fruits are used to feed the poor and needy, those who lack and workers in the house of the Lord including pastors, evangelists, teachers, prophets, apostles and even the brethren.