I’m 18, my goal is long term investing, any advice?

I got into ETFs recently, had some college return money and I put 1.6k into QQQ, VOO, VUG, and IVV ($400 each). I am going to hold these as much as possible as it is almost 100% certain it will go up continuously, as time progresses. Any investing tips? Recurring deposits? Buying more of each when they go lower in value? Anything is greatly appreciated as I am looking to expand my mind with investing.

Edit: New ETF recommendations that I have not invested in will be taken into consideration as well.