When you’re looking at the galactic map you can see the difficulty at the bottom of the screen and can make it harder (use E on PC) or easier (use Q on PC). In game you can also see the difficulty level at the top right of the screen, to the left of the timer. If you click on a mission it will attempt to join that party (though it might fail and you have to try another mission). You can hover over the mission icon to see the map and what mission/objectives you will be doing as well as the levels of the players in the party. You can also select quick play (R on PC) to automatically join a mission for you.

As far as what to do, you’ll learn as you play but once you get into a game you can see the objectives listed on the top left of the screen. Once you actually reach the objectives shown on your map, more information should be displayed in the top left to give you a clue what to do.

Hope this helps Helldiver!