How to do a round railing/ fence I am a very beginner in Blender. I am modelling a tank and want to make a round railing at the top of the tank and a ladder.

The ladder should be no problem, but what is the easiest way to make a round railing with the same curve as my tank?

Here is a photo of the tank:

 

I am not asking for a whole solution, just some tips, since I notice there are lots of features, that makes things easy in blender, but I don't know them at the moment :(...
Please try following steps

Please select the set of edges on the top of your tank, using Alt + click on the edge as show in image below:

Hit Shift + D to duplicate edges
Hit p to seperate the edges
Go to the object mode and you will get a duplicate in outliner
Go to edit mode - Select edges using Alt + click
Extrude a twice as shown in the picture below:

Select alternative faces and hit X and delete Only Faces

You can do it as a single obect too, I hope it help you