Maybe you just went to a stamp store and bought some. Maybe you went through your parents' mail and soaked off the stamps from the envelopes. Maybe you went to the Post Office and bought some new stamps. It has glue stuff on the back that you can't see. The Post Office does that to make sure the stamp won't be used again on another letter.
Some people collect only mint stamps, some collect only used stamps, some collect both.
Look again at the last two stamps. OK, now here is a strip of 3 stamps on an envelope that arrived yesterday. If you want to remove stamps from envelopes, and have permission to do so, cut out the stamp, leaving lots of envelope attached, because you don't want to risk cutting into the stamp. Place what you cut out into a dish of water and wait maybe half an hour or more, until the stamp has floated free from the paper. Now, you have a soggy stamp.