For everything
Make sure your Fancy Editor is OFF. It is important that it is off due to the fact that it has had a lot of problems and causes a lot of the mistakes with these three things.
How to turn off the Fancy Editor:
Step 1: Go to your Profile(Click on your username in the upper left hand corner of your screen)
Step 2: Go to the Settings Tab on your profile

Step 3: Click "No" for when it says "Enable the Fancy Editor (GUI)?

And you're done
Links
Links are really easy with the Fancy Editor Off
Step 1: Find and copy your URL.(Example shown circled in Orange)

Step 2: Paste that URL in your Post

Step 3: Finish your post/Preview and post it.
You end up with
https://www.newschoolers.com
Then you're done!
Pictures
Pictures are just as easy as Links only with one extra thing.
Step 1: Find and Copy your Picture's Direct Link. Make sure that it ends with .jpg or .png or .gif (Example Shown Circled in Yellow)

Step 2: Paste that Link in your post between img codes ([img]imageurlhere[/img])

Step 3: Finish/Revise/Preview Post then post it
You end up with

And you're done!
Videos
Videos are a little harder but make sure you always preview before you post.
Step 1: Make sure your Fancy Editor is Turned off.
Step 2: Go to the Youtube Video Page you want to Embed
Step 3: Click on where it says embed (Circled in green on Example Pic)
Step 4: Change Sizes/Colors to fit your needs.(Optional)
Step 5: Copy the Embed Code (Circled in Purple on Example Pic)
Step 6: Paste that code anywhere in your post.
Step 7: Finish Writing/Posting your post.
Example Picture(Click to make bigger):

And you're Done!
Any questions or problems feel free to ask and I hope this helps.