Staring at a blank piece of paper wondering what to say in your first sentence can be difficult. The way to start your writing process is not to worry about the first sentence of your finished essay, but to start generating ideas. Start by simply listing out the things you enjoy doing. Do you like to hang out with your friends? Are you involved in any sports? Do you like listening to music? Do you enjoy spending time with your family? Do you like reading or watching television or playing video games?
Once you have made your initial list, start thinking about the focus of your essay. You can choose one of two strategies. The first is focusing on one activity you enjoy most and the other is saying that you like to have a lot of variety in your activities.
Once you have made that decision, you have discovered the theme of your first paragraph. If you are writing about a single activity, you can start by saying "My favorite activity is ______" but if you are talking about several different activities you can start by saying "Some of the activities I enjoy are ______".
I think the first answers contain good suggestions, but it occurs to me that you don't have to list everything you like to do. Such an essay would contain a lot of things that would not make you seem any different than millions of other students. You might consider focusing on the one thing that you like to do best and tell what it is and why you like it. The thing you like to do best would probably be the thing that would best characterize you. Teachers assign these autobiographical essays, not because they want to learn all about you, but because they want you to think about yourself and learn more about who you are. The idea of making a list of things you like to do is a good one. But you don't have to include the complete list in your essay. You could pick just one thing from that list and base your entire essay on it. I'm sure it doesn't have to be a very long essay for this type of assignment.
Writing takes brainstorming. Start by making a list of the different things you like to do. After making your list, decide the order you would like to start sharing in your autobiography. Start your first paragraph off by grabbing the reader. For example, tell a story about a time you did something you like but add some humor. Not only will it grab your readers attention but it will motivate you to think of other ways to add voice to your writing.
Writing an autobiography is a way to share your life experiences in a meaningful way. Reading a famous person's autobiography might be a way to get some ideas. Since you want to write about things you enjoy doing, it might be a good use of your time to read an autobiography in that field which may further refine your ideas as well.