Hi there,
I work in advertising company (mainly politics, not video games but hey) so I hope I can help.
- Thumbnail: Try going to the submission tab and keep scrooling down, observe what's catching your eye. Big readable text, contrasting colors, faces and important game elements (sword or a knight for RPGs, Gun for a shooter) etc. You need your players to click on your game first, if your thumbnail looks boring, no one will click on it.
- Short description of your game: Keep stuff simple and short. Don't throw walls of text at your potentional players, they don't like to read, pictures are better.
- Good graphics or artstyle: let's be honest, it's important.
- Spam your game on social media: If you're going to be releasing a game in a month, you should be posting about it pretty much everywhere, everyday. Gifs and videos are the best. If you didn't build an audience it might be rough tho.
- Good trailer
- Try to talk with youtubers and streamers in advance, Don't send them an email one week before release. Plan with them when they should release their video of your game.
- You can always try and find a publisher, there's a lots of them! Make a nice, clean pitch for your game and send few emails around.
- Be lucky
There's more to it obviously but I hope this helps a little.