A Tetris is four lines cleared at once, which can only be done with a straight block. The game generally starts you off with a chance for one, but occasionally the final line will have a hole in it, so make sure it's filled first. There are times where you'll need two straight blocks in order to get a Tetris off the beginning.
Based on this, you can get the achievement just off of the starting formations with a lot of time. If you get a straight block that can't be used for a Tetris, hold it in the top left corner. Additionally, if you aren't averse to using powerups, the Magnet powerup is great for creating Tetrises, as it pulls all the blocks to the left, leaving a good space for a straight block on the right.