Something about significant figures really confuses me, for instance:
If we are counting, say, apples and we say there are 30 apples, there are exactly 30, should 30 be a 2 significant figures number or a 1 significant figure.
And If I am in a phyiscs exams and a quantity like 30 kg is given, this number should be a 2sf numbers but in the same time if say it was 31 kg and I rounded it to 1 sf it would still be 30 and I would say it's a 1 sf number so I'm really confused.
This is a confusion, that's why scientists prefer the scientific notation, which clearly indicates the number of significant figures. If your number is $3.0\times 10^1$, then it has two significant figures; if it is $3\times 10^1$, then it has one significant figure.
As for your question, I would say that $30$ in the case of $30$ apples has 2 significant figures, because here the $0$ is significantly counted.
If this sounds confusing, consider this, you have a metre scale (marked up to $30\ \mathrm{cm}$), then if you measure a length of $30\ \mathrm{cm}$, then it has two significant figures as you counted the $0$ significantly, you are able to count $31$ or $32$ as well! However, if you say, you have measured $3000\ \mu\mathrm{m}$ ($\textit{i.e.}\ 3\ \mathrm{mm}$) then none of the zeroes is significant because the $3$ here is significantly counted, the zeroes are not! Any other person could have measured the same and could have said $3500$! That's why only $3$ is significant!
Significant errors count all the digits measured accurately as well as one additional digit to count the errors. In the above case, the $3$ is the additional digit itself!
Hope this is clear :)