I beat the game!But I can't find the last boss.HELP!!?

To get the last boss, (Metal Sonic)
You need to unlock the last story by getting the 7 chaos emeralds
To get the emeralds you get the key in every other level (not counting boss levels) then finish the level and there is a bonus level where you chase the chaos emerald you need to catch it before it reaches the end.

In addition to The Imperfect's post. If you get hit when holding a special stage key, you lose it, so use Team Rose to get them, as their levels are shorter with fewer obstacles, Badniks, and your Team Blast gives you a Shield and temporary Invincibility.

GJS

I'm with what the other two said, it is important to have all 7 chaos emeralds, but that doesn't completely answer your quesiton, your question was where to find the last boss, to find it, simply enter the story mode screen where you see the 4 different teams, and if you have beaten all of them 100% and also have collected the 7 chaos emeralds, the final mission should be just to the right of the screen, right next to team chaotix, just move the d-pad or thumb pad to the right 4 times and you should see the silhouette of metal sonic with the letters "LAST" in what would look like ice in the label. Good luck and have fun!!

Collect all the chaos emeralds.