It's better to have a good idea what you're going to do before you get in there. Here's another thing you gotta keep in mind, depending on the distance between you and your opponent, there are natural counters that follow certain punches. E.g. say at close range, it's more natural for many to follow a left hook to the body with a left hook to the head. An experienced fighter may anticipate that and duck under your hook, if you can establish that pattern you can set him up for a right hand after he ducks your hook. the possibilities with different punches are endless, but it's the process of planning that matters more than the combinations themselves.

I gotta tell you about a boxing marvel that you may not be aware of. His name was Benny Leonard an awesome lightweight who was the best in the time that his division was rampant with hall of fame talent. He was nicknamed "Benny the Brain" and the "Ghetto Wizard" because of his uncanny ability to outthink his opponents, he was probably the fastest think tank on two feet.

Anyways I'm going to tell you a couple of good secrets that kept him on top. He would say that you gotta talk to yourself, say if you see how a guy commits to his jab you can think how you can counter (slip it and counter with a right/left-hook, etc.) It was his process of planning seeing what you gotta do and practicing that way of thinking that'll put you on a mental level above your opponents. His other secret is that he spend a great deal of time shadowboxing and practicing his defense, through sparring he could sharpen his timing and perception.

Now shadowboxing is a great drawing board to start off with, since you'll find that people have natural tendencies when it comes with countering with certain punches you can put that into play with your shadowboxing. Practice your footwork, counters and combinations and when you're sparring you can take it for a test run so by the time you have a real fight coming up you'll have them all at your disposal because you drilled them into you.

By the way good luck with your sparring later.