When using the plain speech style, (으)라 is added to the verb stem in order to make a command. If you use 하다, the verb stem is 하, and since it ends in a vowel, you attach 라 to get 하라, where you're commanding that someone do something. If you use 먹다, to eat, your verb stem is 먹, and as this ends in a consonant, you add 으라 to get 먹으라, where you're commanding someone to eat. So here, where you use the verb 일어서다, to stand up, you take the verb stem, 일어서, add on 라, and you get 일어서라, where you're ordering someone to stand up.
i googled and 일어나 is wake up and 일어서라 is to stand up. anybody knows the difference?
You wake up from bed (lying position, sleeping) and stand up from sitting position I suppose