Жарко is only used to describe area temperature. For example, 'this room is hot', or, 'it's hot outside.' Otherwise, гaрячe is used to describe object hot. 'The kettle is hot' or 'don't touch the hot stove.'
They are an ablaut pair (similar to 'singing' and 'song' in English). Жарко is the "e-grade" (with palatization of the initial г- due to -e- -ʲa-) and refers to an uncompleted, habitational state of affairs, while горячо is the "o-grade" referring to a completed, resultative state. Both words are cognate with the English 'warm' and the Greek-English 'thermal'.
Does this refer to the body temperature or is it meant in a sexual way (attractiveness) ?
That could be a translation, but literally it is "I am hot". Report if you feel it should be accepted.
Жарко is not an appropriate word. The correct translation for I am hot is Мені спекотно.
Hot weather – Спека