Learn Arabic (Modern Standard): الجو بارد اليوم.

Adding the word for "the" (ال) to "day" (يوم) literally gives you "the day". In Arabic, this specific combination is used as an adverb of time to mean "today".

Can I put اليوم at the beginning of the sentence instead?

Yes! Adverbs of time are very flexible in Arabic. You can put it at the end, like in this sentence, or at the beginning: اليوم الجو بارد (Today the weather is cold). Both sound perfectly natural.

Where is the word for "is" in this sentence?

Arabic doesn't use words like "is" or "are" in the present tense. To say "The weather is cold", you simply put the subject (الجو, the weather) right next to the word describing it (بارد, cold).

Why does الجو have ال (the) at the beginning, but بارد does not?

This is the trick to making a full sentence without using "is"! The subject (الجو) is specific, so it gets ال. The describing word (بارد) is left general without ال. If you put ال on both (الجو البارد), it would just mean the fragment "the cold weather".