Welcome to the T-Shirt Forums!
Your first step should be to read the Private Message that you received when you joined the forums. It will help you find answers to common questions that have been asked in the past.
Then read the Frequently Asked Questions post in this forum section which will give you information about which transfer paper to use and the right equipment you should be using to get the best results:
Frequently Asked Questions - Heat Press and Heat Transfers Forum: New here? Read Me
If you print a design without a box, then you would just use scissors to trim around the design about 1/8 of an inch.