You just need to index the string, just like you do with a list.

>>> 'hello'[3]
l

Note that Python indices (like most other languages) are zero based, so the first element is index 0, the second is index 1, etc.

For example:

>>> 'hello'[0]
h
>>> 'hello'[1]
e

In python3 range is an generator, and not a list. That means that the following code will not require excessive memory:

for i in range(start_pos, len(my_string)):
   print(my_string[i])

If you'd rather use an iterator over my_string then you need to write it yourself:

def iter_starting_at(start_pos, string):
    for i in range(start_pos, len(string)):
        yield string[i]

for character in iter_starting_at(start_pos, my_string):
    print(character)