A line in Illustrator is called a path which is composed of at least two anchor points, a start and an end anchor point.
Draw straight lines using the ‘Line Segment Tool (/)’. Select the ‘Line Segment Tool (/)’ by clicking and holding the ‘Rectangle Tool’ in the tools panel.
Lines segments are “Live”, which means you can edit the length, direction, and angle after drawing them without switching from
the ‘Line Segment Tool’ to a selection tool.
Rotate a line by moving the pointer off an end and dragging.
Once you have selected the ‘Line Segment Tool (/)’ Press and drag. Once you release the mouse you will make the line. (The grey box following your cursor has the length and angle of the line.)
To edit the direction and length of the line move your mouse to one of the ends (or anchor points). Click and drag and you will see you can change the direction of the line.
Press and hold the shift key as you drag, and the line will be constrained to 45°
To move the line, drag it from the center point.
To change how thick the line is you will need to change what is call the ‘Stroke’ in the ‘Properties Panel’ on the right.
You can also change the color of the line by double clicking the box to the left of ‘Stroke’. (The next section will explain more about how to navigate the color options.)
To make the line segment an arrow, click the word ‘Stroke’ in the ‘Properties Panel’.
In the ‘Stroke’ dialog box you can change the weight of the line, direction of the arrow, and make a dashed line.
You can also create an Arc by using the ‘Arc Tool’. In the ‘Essentials’ workspace the ‘Arc Tool’ is under the ‘Paintbrush Tool’ menu.
If you cannot find it in your toolbar remember you can add tools to the toolbar by clicking the three dots at the bottom.
Once you have an arc drawn it can be edited by using the ‘Selection Tool (V)’ or the ‘Direct Selection Tool (A)’.
The ‘Selection Tool (V)’ 
allows you to move the arc and transform it like you would a shape through the selecting points in the bounding box and resizing it. If you hover on a corner of the arc’s bounding box you can also rotate the object with the ‘Selection Tool’.
The ‘Direct Selection Tool (A)’ 
allows you to edit the anchor points of an object. The anchor points are the start and end of the arc (a line or arc can have more than two anchor points). With the ‘Direct Selection Tool’ anchor points can be moved individually to change the shape of the arc. You can also change the curvature of the arc by moving the handles
Anchor points are square boxes and handles are the circles at the end of a line that comes off of the anchor point.
To create a curved shape out of lines you will use the ‘Curvature Tool (Shift + ‘)’. 
Both tools allow you to draw a continuous line by clicking to create ‘anchor points’ you can make as many anchor points as you would like. Once you click the anchor point you started with the shape will close.
You can edit the line you create with the ‘Direct Selection Tool’ by moving anchor points and handles.
Here you can see a shape that was made with the ‘Curvature Tool’. Once selected with the ‘Direct Selection Tool’ you can edit the line by moving anchor points and handles.
Another way to create a closed shape with straight lines is to use the ‘Pen Tool (P)’. 
