Sample Code:

//Here I'm creating a sample txt file.
List<Folder> folderList = [SELECT Id, Name From Folder WHERE Name = 'Test Documents'];
String docFolderId = folderList[0].Id; //Get Folder Id
String fileContent = 'This is a test file.'; //File Content
Datetime currentDT = System.now();
String fileName = currentDT.format('yyyy-MM-dd-HH-mm-ss-SSS'); //Timestamp for unique file name

Document doc = new Document();
doc.Name = fileName;
doc.Body = Blob.valueOf(fileContent);
doc.ContentType = 'text/plain';
doc.Type = 'txt';
doc.FolderId = docFolderId;
doc.IsPublic = true;
Insert doc;