Hi,
I think the best thing you can do is to sort out all the coins, and firstly try and determine precisely what they are. Whether British or foreign, to start with, although some older British coins can be difficult to identify. Sort them into like types, and then into date order.
Selling is a bit of a minefield if you're new to the game, but we can probably give you pointers as to worth and the best places to market them. I will just throw in a word of caution not to get your hopes up to high. That's not to say your Grandfather didn't collect some absolute corkers, but a lot of people come on here with coins that really aren't worth much at all, wondering whether they're worth selling.
When you've sorted, if you'd like to photograph one or two different types and post on here, we can give you some meaningful advice as to the best way forward.