(1) Algebraically the slope between two points `(x_1,y_1),(x_2,y_2)` is given by `m=(y_2-y_1)/(x_2-x_1)`
For example, the slope between (2,5) and (7,12) is `m=(12-5)/(7-2)=7/5`
So you can find the slope of a line if you can identify two points.
(2) Graphically, slope is the ratio of "rise" over "run", where "rise" is can be defined as how much you have to go up or down to get from one point to the next (up being positive and down negative) while "run" can be defined as how far left or right you have to go to get from the first point to the second point (right is positive and left is negative.)
(3) If you are given the equation of a line then you can rewrite in slope-intercept form y=mx+b where m is the slope.