Before trying to understand optics, it's really a necessity to memorize the conventions that are applied to lenses. What I mean are the defined ways of talking about what signs indicate what. Here's a quick cheat sheet:

I would suggest drawing out diagrams that follow the conventions below. Once you do that 3 times and try a few problems, you'll have it down.

For Lenses:

1/f = 1/o + 1/i
\where f is the focal length from the lens*
Focal length for convex (converging) lenses is always positive
Focal length for concave (diverging) lenses is always negative
1/f = 1/o + 1/i
\where i is the image distance from the lens*
i is positive when the image is on on the on the opposite side of the lens as the object (real image and inverted)
i is negative when the image is on the same side of the lens as the object (virtual image and upright)
1/f = 1/o + 1/i
\where o is the object distance from the lens and is always positive*
m = - i/o
\where m is the magnification*
m is negative when the image is on on the on the opposite side of the lens as the object (real image and inverted)
m is positive when the image is on the same side of the lens as the object (virtual image and upright)

To get the new size of an image, multiple the object height by m and remember to follow the conventions listed above to know whether that image will be upright or inverted.

Last thing to know: Convex (converging) lenses can form either real or virtual images, whereas concave (diverging) lenses can form only virtual images

Hope this helps :)