Standard form of a line equation is:

Ax + By = C ,

For your problem:

3x + 4y = 12 is in standard form.

To put it into slope-intercept form, y= mx +b, just solve the equation for y.

y = mx + b, where m is your slope and b is your y intercept.

To go from slope-intercept to standard form, you just need to get x & y on the same side. Do this by subtracting your mx term.