In....what's better : saying sono certo or sono sicura ?......... it's a bit confusing. In certO... is the O ending for I/first person singular... or O ending for a Man saying I am sure?
Same with sono sicurA... Someone above said the A ending is for a Female speaker...
So if it were a Man would we/he say: Sono sicurO?
Which would mean we've got to change endings on those words (and others) depending on our own gender. And we need to do so When? How do we know which words to switch from O to A? Hope this question makes sense, it is a bit convoluted, sorry.
Most adjectives referring to the masculine end in -o; most feminine end in -a. Most masculine plural adjectives end in -i; most feminine in -e. So franwy you are correct that a man would say "sono sicuro". A woman would say "sono sicura"; some men (or a mixed group) would say "siamo sicuri"; some women would say "siamo sicure".