I suggest that you start by reading a picture book to the class. Choose a book with a message/theme that supports the other things you've been doing in class. When you're finished reading, ask your students to draw a picture of what happens next or a picture of their favorite character. After, you can have them present their pictures in small groups.

take a key, such as at and brainstorm all the words the children can come up with that rhyme with at.
once the list is compiled, ask the children to write a simple sentence (4-5 words) using words from the at list.
example:
The fat cat sat on a mat.
A bat went after a rat.
there are hundreds of letter combinations to continue this activity with: ee, ea, ear, etc.

I do not agree with the suggestions on writing something. In first grade it is too much to ask of them to write a story. These babies are still fighting with getting their letters the right way. I would do something on nouns. It is a pretty easy concept to grasp. Watch the School House Rock video on nouns, it's like 35 minutes long. Ask them to name anything that they see and make a list. They just made a list of nouns. Have them go on a noun scavenger hunt around the room, looking for 3 nouns that are big, small, red, whatever. Then give them a few sentences (that you read to them) and have them circle the noun with a crayon. Wrap it up by having them make a mini book of nouns, writing a noun on each page (you'll have to help them spell it) and illustrating it.

You can try to give them activity worksheets or make them watch fun videos.Play games with them like Punctuation Pirates.Where they have to guess what punctuation mark completes the sentence.If they get it right,they get another turn.

Do you have any specific English subject (spelling, grammar, lit.)? I do not think you should make them write an essay, since they are only in 1st grade. You should do something creative like a little art project along with an assignment. For example, you can ask them to write 3-4 sentences on their favorites (thing to do, book, tv show, animal, etc.). Then, have them draw a simple picture with it to describe their "favorite". Have them color i in with crayons/colored pencils. Also, make an example to show them. I hope it works out!

writing a story about themselves