Welcome to modded Minecraft!

We will go back to vanilla Minecraft once 1.17 is stable enough to run a server with.

We are running Ocean Outlast as the modpack and it can be found on the ATLauncher.
You can get the launcher here if you do not have it.

Once you have the launcher you can search for ocean outlast in the packs tab and install it.
When it asks for installing optional mods click the "Select Recommended" button. You can select other mods if you would like.

To help make the modpack run better you may want to change some settings of the modpack. To achieve this you need to click on the "Settings" button on the modpack on the "Instances" tab.
You may want to change the MAX memory to at least 4GB (4096) to be able to run the pack. This pack recommends 6GB (6114) to run smoothly.

Make sure your Java install is upto date and to help java run a little better you can add the following java parameters to the pack:
-XX:+UseG1GC -Dsun.rmi.dgc.server.gcInterval=2147483646 -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M

Once it is installed you can install optifine to get a little more performance out of the mod pack. Optifine can be found here, but make sure to get the 1.12.2 version.
Take the mod and put it in the mods folder of the pack (There is a button "Open Folder" when you are in the "Instances" tab to get to the folder easier).
Then you will need to "add" the mod by cliking the edit mods button on the modpack when in the "Instances" tab, and enable it. Optifine will be installed then.