How to design a unity gain with LM358 op amp?
Create a mid-rail voltage point by using two 10k resistors; one connected to positive supply and one connected to ground. Join the two in the middle and connect to Vin+.

With your input disconnected you will see half the supply voltage on the output of the op-amp. Now connect your input via (say) a 10uF capacitor to Vin+. You should now see the signal repeated at the output.

To use this output signal as an ac source, use another 10uF in series with the output. If you are needing low frequencies 10uF maybe too small.

This TI document has some good circuits with and without gain for ac inputs and biasing. It's called "a single supply op-amp collection".