Rounding to the nearest whole number
If you can round numbers to the nearest 10, it is pretty straightforward to extend this to rounding to the nearest whole number. The basic idea is still the same - thinking about a number line, we are trying to work out which value our number is closest to. The only difference is that we are looking at decimals between two whole numbers.
For example, think about the number line stretching from 15 to 16. The number right in the middle of this is 15.5. Any number below this stays at 15 when rounding to the nearest whole, and any number above this goes up to the next whole number, 16. 15.5 would also round up to 16 by convention (remember, this means that's just what we've all decided to do).
You're only interested in the tenths digit (that's the first number after the decimal point) when rounding to the nearest whole. So if you were rounding 15.354152 to the nearest whole, you would only need to look at the 3 after the decimal point (15.354152) to decide that this would stay at 15 rather than rounding up to 16.