Use a paint stick with roller or pad
Make sure your deck is very clean & dry so the paint will stick, a power washer does a great job. If you don't have 1 you can rent them from Home Depot/Lowes.
We bought a spray painter and painted our deck. We gathered refrigerator sized card board boxes to set against the house to prevent any splatter. We found our sprayer at Menards, they have a wide range of sizes for anyone doing diy or even business spray painting. You can also rent spray painters for a weekend. It went fast and was easy to do top and bottom and sides with more than one coat and it went very well.
Make sure it's deck paint or you'll be redoing it over, next year.
I would power wash the deck first really clean wait for it to totally dry in the sun for at least a day then get Thompson’s water seal and seal the deck up you don’t want to paint it is and you’ll have to do it every year because it’ll chip off but Thompson’s water seal it or feel it nice and then you won’t have to power washing of the year it’ll keep its natural color
If it is pressure treated lumber you need to make sure that it is fully cured. There's a lot of discussion about how long you need to wait. I've heard 3 months - a year.
Clean the decking. If you need to wash, make sure it completely dry before proceeding.
Purchase paint that is formulated for a deck.
Now you are ready to paint.
You can either use a sprayer or roll it on.