In the comic book actual Captain america storyline, Cap is a 1940s world war 2 hero, so how did he get to our time and join the Avengers? During a battle in the 1940s, he fell down to the river and was iced over, supposedly placed unwillingly in suspended animation. Nick Fury and the Shield of our time searched for him, cut out the ice, and thawed him out, just in time to form the Avengers!
This movie trailer post credit comes right at the end of the current Captain America the First Avenger movie. There are even fake trailers at the bottom.