I am writing a story in the 3rd person and i would like to switch the narrator for one of the main characters half way through so that the reader can read how he perceives the story. does anybody have any suggestions on how this can be done without confusing the reader (about who the narrator is)?
Some options to consider:
I'm not quite sure why you think a switch is necessary. Or exactly what kind of switch you're proposing. The title of your question mentions switching from 3rd to 1st, but then the text of your question is more about POV characters, and you can have different POV characters without switching from third to first.
Given that you're mentioning a narrator as if there's an independent voice, I assume that most of the story is written in omniscient third? It's quite possible to 'zoom in' from omniscient to close third (see the Harry Potter stories for an example of someone doing this quite effectively), and you can zoom in to different characters at different times. If you want to make it clear that you're zooming in on someone new, I think the best way to do that is to zoom back out for a little omniscient narration, and then zoom in on your new target.
A VERY rough example:
Jane carefully returned the goblet to the table. There was obviously more going on here than she was aware of, and she didn't want to take any chances until she had it all figured out. [close third POV, zoomed on Jane]
She wasn't the only one looking for more information, although her intentions were certainly more innocent than those of others. While Jane was handling antiques and old ladies, her nemesis was still dealing with Banshees and Necromancers. [omniscient third - and also, total drivel!]
"Get the corpses into the boxes, now!" Dorfman was not a patient man, and one more iteration of the idea that the dead were in no hurry was going to be enough to make him start adding new corpses to the waiting pile. [close third POV, zoomed on Dorfman]
This example is a bit heavy-handed (the modern equivalent of 'meanwhile, back at the ranch'), but hopefully it gives you the idea, and as I said, you can find more subtle examples in Harry Potter and elsewhere.
Is this what you're asking, though? It's a way to switch POV characters, but it doesn't switch to first person. I just don't think it's generally necessary to make that sort of change...
I assume you want to move from 3rd to 1st person. Is this correct?
You could have an incident in the 3rd person, like the hero goes to a bank to check his balance and later meet his girlfriend Suzy. Briefly have the hero making plans about this in the 3rd person:
"He was thinking of going to the bank, so he could withdraw some money to take his girlfriend out to that fancy 5 star hotel."
Then you could switch to 1st person in the next chapter:
"I walked into the bank. It wasnt too busy, which was great, as it meant I could spend more time with Suzy later"
This way the reader will know you are talking about the same character, you have just moved from 3rd to 1st person.