In base ten, the different places are, from right to left, 10^0 (ones place), 10^1 (tens place), 10^2 (hundreds place), etc. There are ten digits, 0-9, and each digit can occupy any place, indicating a different value (e.g. a 9 in the tens place is a different value from a 9 in the ones place).
In base two, the places are, from right to left, 2^0 (ones place), 2^1 (2s place), 2^2 (4s place), etc., and there are only two digits that can be used in any place, 0 or 1, thought they are also used to indicate different values (e.g. a 1 in the 4s place represents a bigger value than a 1 in the 2s place). Let's say you have the number 1110 in base two; that's a 0 in the ones place, a 1 in the twos place, a 1 in the fours place, and a 1 in the eights place. 0+2+4+8=14, so that number would be 14 in base ten. In base sixteen, that number would be written as a single digit, E.
In base 16, the places are, from right to left, 16^0 (ones place), 16^1 (16s place), 16^2 (256s place), etc. The numbers from 0-9 are the same, and the numbers equivalent to those from ten to fifteen are written as A, B, C, D, E, and F. Let's say you have the hexadecimal number 3C. What is that in decimal? You have a 3 in the sixteens place and 12 in the ones place; 3*16 + 12*1 = 48+12 = 60.
To convert from binary to hexadecimal, group the digits in the given number by fours. Treat each group as though the digit on the right is a ones digit, the next digit over is a 2s digit, the next is a 4s digit, and the one farthest on the left is an 8s digit. If you have the binary number 1110, for instance, that's one group of four digits. In the ones place, you have zero, so you aren't adding one. You have a 1 in the twos place, so that's 2. You have a 1 in the fours place, so that's 4. You have a 1 in the eights place, so that's 8. Add up everything you have; 0+2+4+8=14, in base 10. Remember, though, you're converting to base 16, which has up to F in single-digit numbers (F=15, in base 10). 14 is one less than that, so it's E. A pair of digits is kind of irrelevant when converting between binary and hexadecimal, unless it's the last two digits on the left, left over after grouping the number into sets of four digits.
Given the hexadecimal number FF, that's an F in the sixteens place, and an F in the ones place. Remember that F is equivalent to fifteen, in decimal. So that's 15*16 (for the sixteens place) + 15*1 (for the ones place), which equals 255 in base 10.