The easiest way I have found to start a new article is to first create a link to it on the Main Page -- the page you first come to after logging in.
Look on the main page as to where your article fits and then use the edit tab to create your article link. You do this by using 2 opening "square" brackets ("[[") in front of your article name and then two closing brackets ("]]") at the end of your article name. Of course you need to put your article name in place of the example I used. Once you have done that save the page with the save tab on the bottom.
Now that you are back at the main page in the non edit mode find your new link (it should be in red, meaning its empty) and click on it. It will take you to a blank page that says "Editing ..." You can now begin writing your article.
If you want to bold something highlight the text you want to make bold and click the "B" tab at the top of the page, likewise for italics.
Images are pretty easy to upload. The max image width you should use is 800 pixels and just keep your file size modest -- if it is too large it will tell you. The easiest way I have found to create an image is first to insert, using the picture tab at top, where you want the picture to go. Then replace the word "example" within the tag with the name you want to assign the picture. It does not have to be the same as the picture's file name on your computer. Once you have done that then save your article. Now, click on your new picture link and it will take you to the upload section. You will see the name you gave your picture for the wike and above that box is the browse button to access the picture you want to upload. Use it and insert the file name of the picture you desire. Then you are ready to upload using that button. Once it is done you will see a preview. Now all you have to do is go to the main page, then to your article link, click on it and your picture should be there.