how can i use "有点儿“ and 一点儿

我有点儿饿。（有点儿+adj）
你喝一点儿水吧。（V+一点儿）

有点(儿) usually placed after a subject and before an adjective. It is usually used to express negative feelings or impressions.
一点(儿) is always placed after an adjective or a verb, and it cannot be placed before an adjective.

一點兒refer to quantity. 有點兒 go with adjective.

“有点儿”it means that the amount is not large; it means a little bit; a little (mostly used for unpleasant things);“有点儿”can be used as a predicate while “一点儿” doesn't have this function.
“一点儿” indicates an indeterminate amount, or a small or very little amount;
Sentences to help you understand the differeneces:
动词verb+一点儿+名词noun: 喝一点儿水。
形容词adj+一点儿: 昨天20度，今天10度，今天冷一点儿。
有点儿+形容词adj/or verbs like"想、喜欢、希望": 今天有点儿热。我今天有点儿想去看电影。

"有点儿" is an adverb of degree, such as: "有点儿咸", "有点儿贵", "有点儿不开心", put it before the adjective.
"一点儿" can also express the degree, but it usually means negative, such as: "一点儿都不好", "一点儿都不开心"; the second usage, often placed after the adjective, expresses the degree , such as "快一点儿" and "轻一点儿".

v+ 点（儿）means kind of ， a little
有点儿=有一点儿=有一点
我有点儿饿。我有一点儿饿。我有一点饿。

一點兒：數詞，買一點兒水果。
一點兒和有點兒都可以跟形容詞發生聯繫 ，表示程度不高的意思，但是用法完全不同。
有點兒＋形容詞/這件衣服有點兒貴。
形容詞＋一點兒/這件衣服貴了一點兒。