Learn German: Heute ist es sonnig.

In German, the main verb must always be the second idea in a normal statement. Because this sentence starts with the time word Heute (Today) in the first position, the verb ist (is) has to come next to stay in position two. The subject es (it) then gets pushed to the third position.

Can I start the sentence with es instead?

Yes, you can say Es ist heute sonnig. If you start with Es, the verb ist stays in the second position, and heute simply moves after it. Both versions mean the exact same thing, but starting with Heute puts a little more emphasis on the fact that it is happening today.

Does es work exactly like the English "it" here?

Yes! Just like in English, German uses an impersonal es (it) as a placeholder subject to talk about the weather. Since no specific person or object is doing the action of "being sunny", es steps in to make sure the sentence has a subject.

Is sonnig related to the German word for "sun"?

Exactly. The noun for "the sun" is die Sonne. Adding the -ig ending is a very common way to turn a noun into an adjective in German, just like adding a "-y" turns "sun" into "sunny" in English.