How does lightning from a cloud?

The cloud has an upper part that has a positive charge and a lower part that has a negative charge. The electrons in the ground move away from the cloud because like charges repel. This causes the electrons from the cloud to jump to the ground quickly in an effort to make the ground neutral, this is what creates lightning, kind of like a spark. Hope this helps :D
In cloud to ground lightning, it may seem that lightning strikes in a downward direction because of how quickly it happens, however, this is actually inaccurate.

By law, negative charges reside at the bottom of a storm cloud.

Positive charges reside at the top of a storm cloud.

If there is enough electrical energy within the storm cloud, a single negatively charged particle (electron) will emerge from the cloud and begin heading towards the ground. This is called a "stepped leader".

On the ground, positive charges begin building up below the stepped leader and since opposite charges attract each other, these positive charges begin heading up to meet the negative stepped leader.

When they connect to each other, a channel is established and more electrons begin flowing downward.

Next a "return stroke" of positive charges flow up this channel and create the bright flash we see as lightning.