The best thing to do is either to find a common denominator or convert them to decimals.
I will find a common denominator.
A common denominator of 3 and 5 is 15.
You multiply the first fraction by 5
2*5= 10
This then becomes 10/15
You then multiply the second by 3.
4*3= 12
Therefore, this becomes 12/15
As the second of these fractions is larger, this means that 4/5 is larger.
The alternative method is to put them into decimals, in which case 2/3= 0.66 and 4/5= 0.8
Hope this helps :)