So, I have 2 methods I use. First, if I need to edit the video (i.e. my iPhone was upside down so the video is flipped), I use Windows Movie Maker. I don't know if that is standard on PCs, but it was on mine. There may be other services online. It also lets me trim down the sides if I want to cut things out, or merge multiple clips into one.
Then if I want to upload the whole video with sound, I use Youtube. Make sure you have a (free) account and then click the little camera on the top right to "create a video". You can change permissions to be private, unlisted, etc. After it uploads, on the video page, click "share" and a box will pop up. Copy the link, then on the Dis post click on the film strip icon. It's the one in the box as the Emojis and photo icon. Paste the link into there and click embed and then it will go into the post.
Second way is if I just want to show a short clip and don't care about sound. I make a gif. I use giphy. You may need to also have an account, but it's free and simple enough. On the top right click "Create". Upload the video you want to post, then click through and you can change the length of the gif, the start time, add stickers, privacy, etc. Once it is all created, I go to "copy link", copy the link in the "gif link" line, go to the Dis post, click the Image upload icon at the top (next to the emoji icon), and paste the gif link, which imbeds it into the post.
Hopefully that helps! I'm sure there are other methods, but these are the ones I've found easiest