Hello. I am just figuring out inkscape. I have watched many tutorials on YouTube and read multiple forum posts. I apologize if this question has been already asked and answered. 

I used the bezier pen to trace outline of a truck. I am now adding additional features to the outside outline. How do I attach a new line or circle to the existing outline? 

I attached a picture for reference. I want to attach the railing to the bed of the truck.

Thank you so much!

Select what you want to merge and go Path->Combine.

When I do that, it makes the lines thicker. Is that what it is supposed to do? 

Also, how to I remove the part of the arc that is connected to the double lines going horizonal and vertical? I know how to remove in BobCad, but wanted to know if can be removed in Inkscape too.

There´s just one Style per Path aka Stroke/Width/Fill color possible. If you want to keep different Stroke width for example you may consider "grouping) instead.

2. If you make the bars from Rectangles you can go Path->Union: