Hi, i think my business will turnover more than 75 this financial year, when should i register for gst? and can you tell me if my understanding is correct

if i bought an item and it included $10.17 GST and i sold it for $139.95 paying $14 GST would that mean i pay the difference between the two figures? which would be $3.83

OR do i pay the whole 14 dollars now but come tax time i get alot back?

Just not sure how the whole thing works like when it is paid, how it is calculated etc. Any help would be great thankyou