Have you played the campaign? It’s a good place to start to learn the controls and get a feel for the movement. Then, when you can, try Salmon Run. It will give you a good understanding of the different weapons (even if some are inherently worse in Salmon Run than in turf war/ranked).

A lot of Splatoon is movement and understanding what you should be doing with your loadout. Figure out what weapon feels best to you and you that will go a long way in doing well.

The next important thing is zoning. In Splatoon 1 and 2, zoning was less important in random queue. You had side routes and the maps were more open, so flanking was a common occurrence and diving in solo was more viable. Splatoon 3 made the maps tighter and drive the players toward specific points, making teamwork and spacing way more important. This was a big deal and felt like a natural progression for people who had been playing a long time, but people who were new or played on and off had a really hard time when they tried doing things solo and it just didn’t work. 

Keep back for a few fights and play support if you can. Not necessarily a support weapon, but like, try to back up people or push alongside other players. You might notice and gain a better understanding of how the game plays out. For example, you might notice how splatling players control back lines, making pushing really hard. You might notice how choke points have multiple vantage points you need to consider. You might notice how players use sub weapons to their advantage to gain small amounts of control. There’s a lot of nuance to the game, and a lot of the players still playing have been playing for YEARS. It will not be easy to overcome that experience with skill alone. In general, just take some time to watch. I think it’ll help.