ok, you must have a shovel, ok I find one but I can't "take" it... I can "grab" one but how do you actually use it to dig up a grave?

You can find one at the Goodsprings source, buy one from Chet in Goodsprings, and maybe at the Crimson Caravan. You just need it in your inventory. You can give it to your companion then grab it when you need it, as it takes up 3 lbs.