OK, So, I will try to help you as best as I can. These can be difficult.
Start our your first saying how excited you were to go on this expedition. Next, write Lewis or Clark. Your next entry should be about what the other may have done that day. Next Entry should be Sacajawea, talk about you met her and how she can speak two languages. Next should be about York, I didn't really learn about York, but talk about he came along, how he helped, all that. The Lewis and Clark expedition lasted from May 14, 1804 to September 23, 1806. Your dates can be anywhere in between there. If you are unsure about some information, check in your book (if you have one), or look it up on the internet.
Make sure to have fun, and be creative Hun, hope this helped. Have nice Day/Night.