Indian voices of the Great War: solders' letters, 1914-18 (in Tripod)
Porter, steward, citizen : an African American's memoir of World War I (in Tripod)
Look at this search example: "World War, 1914-1918" AND ("personal narratives" OR "correspondence" OR "diaries" OR "sources" OR "notebooks")
Break the search down into two components: Subject/Topic + Genre
Subject/topic that you're interested in: "World War, 1914-1918" AND ("personal narratives" OR "correspondence" OR "diaries" OR "sources" OR "notebooks")
Genre terms (the types of sources you want): "World War, 1914-1918" AND ("personal narratives" OR "correspondence" OR "diaries" OR "sources" OR "notebooks")
How do you get subject terms? Look at books in Tripod for ideas - each one will have subject terms listed. (For example, "African American soldiers" is a subject for this book.)
What other source types can you find? See this list of primary source genre terms for ideas.