Numbers in Japanese are pretty complicated and varied. You will need to know both on'yomi and kun'yomi readings of each numeral, on top of specific counters which change the pronunciation of the base number. Some of these quirks and variations have patterns and others don't, ultimately you'll just have to learn these things as you go.
This is ignoring some of the quirks with incredibly large numbers I don't have any experience to remark on.
Here's a video that might help. It's not super beginner friendly, but I thought it was interesting, so I'm sharing it. Also check the guy's pinned comment for counting up and down.
I'd say focus on the numeral for counting first, then start spreading out to months, days etc.
A comparison I'd make to English is how we use prefixes like "tri" and "bi" and "quad" etc...in a similar-ish way, Japanese numerals may be said differently when combined with another word.
