Basically, use lange if the word would use de (common gender) and lang if it would use het (neuter gender). Always use lange if there is a definite article.
een lang meisje
een lange vrouw
het lange meisje
de lange vrouw
(de) lange meisjes
(de) lange vrouwen
Always leave adjectives uninflected (no e) if they're neuter indefinite (een lang meisje) or if they're in the predicate (De vrouw is lang, ze zijn lang).
There are many rules for this, depending on the gender of the word, if you are talking about plural or not, if there is a verb, 'a/an' or 'the' before the word or if it is used independently. Try this link, it is also at tips and notes I have been told:
A person is usually called "tall", but a thing is usually called "long". Even the person's arms and legs would be called "long". I have heard a person called "long and lean", but that is an expression. Giraffes are often called "tall", but an alligator or snake is called "long". Actually, "tall" is used for vertical height. Buildings and trees are also called tall. "long" is usually used for horizontal length, but it can mean any length.