l is a list of strings. When you put numbers between single quotes like that, you are creating strings, which are just a sequence of characters. To make your code work properly, you would have to do this:
l = [-1.2, 0.0, 1] # no quotation marks
x = 100.0
for i in l:
if i < x:
x = i
print x
If you must use a list of strings, you can try to let Python try to make a number out of each string. This is similar to Justin's answer, except it understands floating-point (decimal) numbers correctly.
l = ['-1.2', '0.0', '1']
x = 100.0
for i in l:
inum = float(i)
if inum < x:
x = inum
print x
I hope that this is code that you are writing to learn either Python or programming in general. If this is the case, great. However, if this is production code, consider using Python's built-in functions.
l = ['-1.2', '0.0', '1']
lnums = map(float, l) # turn strings to numbers
x = min(lnums) # find minimum value
print x