Can a cleric not have a god?

No, a cleric doesn't need to worship a god. Eberron has a couple religions that don't have any kind of divine entity at all, and the gods of that setting (probably) don't even actually exist.

Read more

Regarding this, can clerics have children?

Its canon 33 decreed: "Bishops, presbyters, deacons, and others with a position in the ministry are to abstain completely from sexual intercourse with their wives and from the procreation of children. What school of magic is teleportation? Teleportation is the second most famous use of arcane magic school called transmutation. It is taught by the mages of Kirin Tor in Dalaran. Magi use their knowledge of the arcane to bend the fabric of reality to circumvent the distance between two points.

Regarding this, what is the best spellcasting class?

Spellcasting is one of Dungeons & Dragons' more complicated elements, but Warlocks are by far the best class for it. Is cleric or paladin better? They fill different roles. Paladins are good at burst damage, by using its smite ability twice every round. Clerics are good at sustained damage, thanks to spiritual weapon (and similar spells). Paladins can have better AC and more HP, but clerics get better healing.

And another question, what's the difference between clerics and paladins?

The paladin makes themselves worthy of a god's blessings while a cleric is deemed worthy by their god. It's a subtle but important difference. A cleric might not know why they are chosen and are rarely given a singular purpose. Rather, they exist as vessels for their gods to manifest their power and will.