Hello,
you have to create an image object (tshirt) with a text object that holds the message.

1. to do that , load the image with fabric.Image.fromURL() function and inside the function , also create a text object that is going to show the tshirt message.
   so, your image and text belong to a group object.


2. every time you want to load new text , you call the loadText function and you change the text object.


3. i also added 4 buttons in order to manupulate up/down/left/right the text .



4. you can export the canvas + image+ text into the function saveImg(),
   but on the jsfiddle you will get a security message for Tained canvases.
   that happens because on the example i load the image from another domain and the code runs on another domain, you can run that code on your web application with no problem at all.




5. that is the code :

   
   
   

    var canvas = new fabric.Canvas('c');
   
    var scaleFactor=0.4
   
   
   
    canvas.backgroundColor = 'yellow';
   
    canvas.renderAll();
   
    var myImg = 'http://izy.urweb.eu/files/tshirt.jpg';
   
   
   
   
   
   
   
    fabric.Image.fromURL(myImg, function(myImg) {
   
   var img1 = myImg.scale(scaleFactor).set({ left: 0, top: 0 });
   
   
   
   var text = new fabric.Text('the_text_samplenand more', {
   
               fontFamily: 'Arial',
   
               fontSize:20,
   
           });
   
   
   
   text.set(top,myImg.height*scaleFactor-myImg.height*scaleFactor+150);
   
   text.set(left,myImg.width*scaleFactor/2-text.width/2);
   
   
   
   var group = new fabric.Group([ img1,text ], { left: 10, top: 10 });
   
   canvas.add(group);    
   
   });
   
   
   
   
   
   $('#loadText').on('click',loadText);
   
   $('#saveImg').on('click',saveImg);
   
   
   
   function loadText(){
   
     console.log($('#logo').val());
   
      canvas._objects[0]._objects[1].text = $('#logo').val();
   
      canvas.renderAll();
   
   }
   
   
   
   
   
   function saveImg(){    
   
   console.log('export image');
   
    if (!fabric.Canvas.supports('toDataURL')) {
   
     alert('This browser doesn't provide means to serialize canvas to an image');
   
   }
   
   else {
   
     window.open(canvas.toDataURL('png'));
   
   }
   
   }
   
   
   
   $('#left').on('click',function(){
   
   canvas._objects[0]._objects[1].set('left',canvas._objects[0]._objects[1].left-1);
   
   canvas.renderAll();
   
   })
   
   
   
   $('#right').on('click',function(){
   
   canvas._objects[0]._objects[1].set('left',canvas._objects[0]._objects[1].left+1);
   
   canvas.renderAll();
   
   })
   
   
   
   
   
   $('#top').on('click',function(){
   
   canvas._objects[0]._objects[1].set('top',canvas._objects[0]._objects[1].top-1);
   
   canvas.renderAll();
   
   })
   
   
   
   $('#bottom').on('click',function(){
   
   canvas._objects[0]._objects[1].set('top',canvas._objects[0]._objects[1].top+1);
   
   canvas.renderAll();
   
   })
   
   




6. that is the jsfiddle example: http://jsfiddle.net/tornado1979/zrazuhcq/1/

hope helps, good luck.