1. draw random line. eg: from top to bottom art board
2. this is your end point. its important to know where is your "end point" of the line since what AI does is joining the both open "end point".
3. ok now draw another line. eg. straight line, we're gonna join this line/ path with the previous one
4. now this is the second "end point" of your new line/ path
5. Select all the path Objects
6. now after selecting both path go to
Object > Path > Join
or you can use Ctrl+J (windows OS) or cmd+J (mac)
or you could simply right click your mouse and choose Join
you will see the both end point now joined automatically
7. after the end points joined each other now you will have two new end points, you could continue joining other line with the new path Object
that's it... piece of cake huh?
good luck, and have fun
Teedz