From this page of our guide to Sniper Elite 4, you may check where to find and how to secure the checkpoint for your first mission in the game - San Celini Island. Visiting the checkpoint is an optional mission objective, but it's not worth skipping it, because it's an opportunity to get more XP.
Your task is to clear the control point (M1.1), located on the way to the entrance of the cave. The safest path leads through the ruins (see chapter eliminating the officers: the second officer). This way you can start eliminating enemies from the inside. There is only one soldier in the cave (and a small ammo deposit).
Get rid of two soldiers by the entrance. If you want, you can use rocks to lure them out and kill them silently. You can also kill the soldier on the left with your pistol, and approach the one on the right in crouching position (his helmet may protect him from bullets).