The Roman Numerals on the bottom are often used instead of the "real" name to make it easier to change keys (for instance, if the key is too high or low to sing). The important thing is that these names are relative, that is they are the same TYPES of chord whatever major key you wish to use. That is why the roman numerals can come in handy.
All being well, you should be playing something like these chords:
A better understanding of this can be achieved by comparing the above with another key. This time, choose the notes of D Major:
Although the notes have changed, the step pattern of the Major key is the same (see section on Major Keys) and therefore the same Roman Numerals can be used:
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| D | Em | F#m | G | A | Bm | C#O | d |
| I | IIm | IIIm | IV | V | VIm | VIIO | I |