Once you've recorded your TikTok video, there should be the Aa symbol at the bottom of the screen. This is where you can enter text.
You just need to tap the Aa symbol, and then you will be prompted to enter your text. You can also change the font and size of the text.
Once finished, select Done. You will be able to set the duration that you text lasts on your video, and decide when they appear in the video. So you can enter one piece of text at the beginning of the video, and another at the end if you wanted.