Example :

Hello ! I'm a begginer with Inkscape and I have trouble with the software. (also sorry for my bad english)

I create various object point to point but when I join the last point to the first the line double and I can't fill it with a color.

I want to create object like the triangle on the left but with various form with the "Bezier curve and Straight line tool"

Hope to find a solution, thanks for your time.

Make sure you have "None" as shape selected:

Hello ! Thanks for your anwser ! It work ! Was in ellispe.

I have another question regarding this option. The 'none' option mage the line 'flat' I like how the ellipse option add thickness to the line any idea in how I can recreate this effect ?

There's a Live Path Effect called Power Stroke which lets you add some dynamics to your paths:

Thanks !! Love it it'll definitly help me, I have another question because I don't have the exact effet you have in the GIF

When I have a closed object (the triangle in the exemple) power stroke add thickness in equal amount in the whole object. How can I choose to add thickness only on one side of the object ?

Thanks again !

Switch to the node tool. Then you should be able to see different handles. You can tweak the effect with those handles. 

Hi

But closed objects like the left triangle I have only one "node" to ajust the power stroke. How can I add more ?

Hover over one control point with the Node-tool and check the Status bar at the bottom of the Inkscape window to get hints and instructions. ctrl-click on an existing control-point will add one on top of it - so you have to move it - while ctrl+alt+click deletes one control-point.