People say this to mean that an ambitious person is more successful than a person not trying to achieve anything. Originally it meant the opposite and was critical of people trying to get ahead.
Things, especially education, that affect and influence us in our childhood shape the kind of adult we turn out to be. (There are various versions of this, like 'As the twig is bent, the tree's inclined' and 'As the twig is bent, so the tree inclines', 'As the twig is bent so is the tree inclined')
The last straw is the final problem that makes someone lose their temper or the problem that finally brought about the collapse of something. It comes from an Arabic story, where a camel was loaded with straw until a single straw placed on the rest of the load broke its back.
If someone leads you up the garden path, they deceive you, or give you false information that causes you to waste your time.
'Lead someone down the garden path' is also used.
If someone has lost the gourd, they are out of the mind or have gone crazy- "gourd" is a melon-like plant that symbolizes a person's head. ("Out of your gourd" and "Off your gourd" are also used.)
If trying to find something is like looking for a needle in a haystack, it means that it is very difficult, if not impossible to find among everything around it.