Once you're logged in just look for a button that says "Start a new hub". Usually it's located somewhere at the top of the screen. If you're not familiar with the upload interface, feel free to play around with it for a while. HubPages uses capsules and to add new ones you need to click on the correct one on the right hand side. Text is the capsule you will use most, but if you have pictures you would like to add, that is also an option (among many other things).
If you're unsure about the quality of your writing, feel free to ask (within the hub) for helpful feedback. Don't worry about posting something with errors because you can continue to edit it and change it as much as you want (or delete it if you feel it doesn't properly reflect you.)