I've just had dinner ( the food is still in my stomac

1.an action was completed/finished in the past but still extended to the present. For example, 

     ---  I have studied English for 10 years (you studied in the past but you are also studying)

2. You have just finished something but the result is still there.

    --- I have just cried. (The result is that your eyes are red)

However, if you want to use since, for, already, just , ever, yet, so far, lately etc. , you can actually use them with this tense :)

Good luck and have a great day :) 

We use the Present Perfect to say that an action happened at an unspecified time before now. The exact time is not important. You CANNOT use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc. We CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc.

http://www.englishpage.com/verbpage/presentperfect.html