Firstly, may I ask; How do you know you do not have a good voice?

Secondly, like everything in life the finished product is made up of a number of key elements all of which must first be manufactured and then pieced together to combine in the finished article.

A good singing voice cannot be manufactured. However a mediocre voice can be improved to an acceptable level, just as a good singing voice can be improved with technique and practise.

First discover if you have a natural ear. That is; can you memorise a simple tune and then sing or even whistle it correctly.

If you can carry a tune here are a few steps to help you get started.

Learn to pitch the basic notes within your vocal range (do re me so fa la te do) etc. Going up the scale and then back down again.

Once you can pitch these notes correctly, next comes breath control.

Practise the scales above holding onto each note for 4 seconds without going off pitch.

Next, expression. The voice is an instrument. The singer must use it to convey every nuance the song writer had in mind and maybe then some more as well. Listen to really good singers and hear how they create emotion, and feeling in a song by using expressive vocal techniques. Start simple then build a range of techniques with which you can colour your singing.

Find an easy tune you know and like to sing. Having practised and mastered these basic steps use them now and sing [within your own vocal range - not necessarily the key in which the song was performed] that song you like so much and with expression, empathy and love. Good Luck.