Getting a sim card for iphone on tmobile?

I'm going to buy an iphone 4 from a friend and i plan on using it on tmobile.right now i have a samsung smartphone that uses a regular sim card. I was wondering can i buy a micro sim card from tmobile and transfer my current plan to that sim, or do i need to set up a whole new plan? If so would cutting my sim card just be helpful? Or is there any other solution without touching my current plan?

If you call t-mobile they will send you an iphone sim card with all your account on it, then they just cancel your current sim. You can cut your sim but be careful.look on youtube for videos that show you how.