Ask Questions
Okay, so you've wrote your first sentence. But what next? What should I write? Well, think about that first sentence. Say it's (and this is completely random), "The warm summer air seemed choking as I trudged up the hill". Ask yourself, who is trudging up the hill? Why are they trudging up the hill? Do they think they will gain something from trudging up that hill? If yes, what? Does your character even want to be trudging up the hill? How do they feel to be going up that hill? Is somebody traveling with your character? Who? What do they think of the situation? Think about when that character gets to the top. Is there something up there? What is it? Did the character expect it to be up there? Or are they taken by surprise? Or is it someone who's up there? Good guy or bad guy? Describe them. What does the character do now? Go back down the hill? Take the animal/person/artifact and go down? Just sit at the top of the hill for a while? Describe emotions.
Really, the possibilities are endless! Just ask yourself, "What next?"