There are lots of other word pairs with an -s- and -c- (for example, practise/practice) that are differentiated by the fact that one is a verb (the -s- one) and one a noun (the -c- one). This one gets mixed up just as much, so it’s time to talk about it!

Advise is the verb. It means to offer advice to or to recommend. “‘I would not advise you to climb the steep face of the mountain: walk up the slope, instead,’ said John.”

Advice is the noun. It’s what’s advised: a recommendation or guidance offered with respect to future actions by the person being advised. “We took John’s advice, and took the easy way up the mountain.”

It should become easier to remember these once you have established that (in UK usage):

The word with the s is the verb, and involves doing something

The word with the c is the noun, and involves the thing itself.

You can find more troublesome pairs here and the index to them all so far is here.