There's a slight difference between those words. You use "sicher" when you're (almost) 100% sure that something is going to happen. e.g. Meine Mutter wird sicher sauer sein. (There's a high chance that she's going to be angry.) However, you use "bestimmt" when you suspect something. This suspicion doesn't really have to come true but still, you're being optimistic about it coming true. e.g. Das Geschenk wird ihm bestimmt gefallen. (There's a chance that he might not like the present.) Do you understand what I mean? :)