What have you been feeding to this point?
I recommend the simplest, a three part brand, Fox Farm or GH flora, and follow the directions on the label carefully for best results.
The reason I mention these two brands is because lots of people use them, so any issues you might run into with them likely somebody's already ran into and you'll be able to solve issue quickly!
Right now you want to be feeding transition nutes because you have transitioned into flowering, you will feed transition nutes for approximately two weeks to one month depending on how plant responds