Dont forget to throw kicks, if he dosent come forward for you to counter, kick the bitch.
Well, if we're gonna break boxing
rules, might as well suggest some knees and elbows while we're at it, eh?
Let's see if I can give some advice that's a little less cliche...
Alright, so you're taller; everyone has already suggested that you use your jab... But I bet that, since you're untrained, there's a pretty fair chance that the guy may just wade through your jabs and get on the inside. So, here's how you can stay on the outside:
whenever you're done attacking, step back at an angle. When he's attacking you, cover your head and step back at an angle. If you retreat in a straight line, he'll catch up to you if he's determined. If you go off at an angle, he'll have to turn before he can pursue, which will give you the extra split second that you need.
That's the only unique (thus far) thing I can suggest. But I'll also throw in some of the good ol' "throw more than one punch at a time," "when throwing combos, vary the height of your punches," "keep one hand in front of your face while the other punches," and whatnot.
Oh, and I know a good one! Whenever you throw a jab, step forward with it; that will get your entire bodyweight behind your punch. If you aren't good at putting weight into your punches (which is most untrained people), stepping into your jabs will make them your strongest attack...
Of course... you need to train regularly to work these into your habits... so when you get hit, it'll probably all go out the window.
In which case, AtomDanger's advice is probably the best.
(but seriously, if you can remember to step off at an angle, it'll make a world of difference)