Can this have the sense of 'I will come soon' since 'I'm coming soon' in English has the sense of future.
Yes, it does when 'snart' is added. If you want to specify that it's very immediate, you could use 'straks' instead. "Jeg kommer straks" (I'll be there in a second)
Why does " ; " stand instead " , " ?
A comma would be incorrect because the two parts of the sentence can stand alone. If it said "I am coming soon, but I need to eat first." you'd use a comma because the second part is not a complete sentence.
The semi-colon (;) usually implies that whatever is coming behind it is going to explain the first part of the sentence. In this case, the writer is explaining why they're coming "soon" and not "now".
When does one use først instead of første?
If I'm not mistaken, første would mean THE first, it's a definite form.