If I'm interpreting you correctly, then the best way to do it would be to have a for loop, which would go through 20 times, and input the iteration number address of the array. To work out the highest, you can either do it as you are going, by setting the highest to the first number you input, then every time you input something, you check if it's bigger than that, if it is, set it as the highest number and continue. If you want to do it all at the end, then the best way to do it I can think of is to really do the same thing.
Do a search on this site, and you'll find that a lot of beginning programmers have asked for help doing just what you're asking, and have been helped. Look at those answers, and you will be helped as well..