All of these are possible. a) I did it! b) I've done it! c) I made it! Tense-wise, the two past tense constructions seem more natural, as they suggest that the difficult journey is now over and done with. 'I did it!' is quite a general statement, and might describe more than the actual journey. For example, it might be the first time that a young person has driven alone on a motorway, so it refers to particular achievement on the part of the speaker. 'I made it' refers to the achievement getting through the journey and arriving home, which is more specific.