youtubeに投稿したよ！って英語でなんて言うの？

The act of putting a video on YouTube is called "uploading." You can use it like this:
- I uploaded a new video.

You can also call the video an "upload."
- Check out my newest upload!

• You can also use this for pictures or anything posted on SNS.

• I uploaded those pictures we took this weekend. They're on Facebook.

  Some phrases you can use when you want people to look at your new uploaded video are:

• Check out my (newest upload)!
• Take a look at my (newest upload)!
• Click the link and check out my new video here: (link).

If you post something YouTube and you want people to watch it, you can say:
Check out my new video on YouTube!

Check out
- This is an informal and common phrase used to ask something to watch or see something.

You can also say:
Watch my latest video on YouTube!
'Watch' in this sense is used as a verbal command to look at something
Latest - The most recent.

I hope that helps!

"Check out my post on youtube!"
This tells people they should look at your post/video/photo that you put on youtube.

"Check out my post on youtube!"
This excitedly tells people to look at your youtube post.
"check out" means look at.

"I uploaded a new video on youtube, check it out!"
This excitedly explains to people that you have a new post on youtube and encourages them to go have a look.

When we load video data to youtube...we are "posting a video"
Like "posting a letter"...Except its digital that we have sent it off into cyberspace.
Potentially to be seen by the whole world ...(if it goes viral)

Posted a video means the same thing as uploaded a video.
Check out is informal English.

If you 'check out' something it means that you look at it carefully - possibly for a specific reason.
Eg. "We need to stop off at the railway station to check out the timetable for trains on a Sunday."