⇒ To arrange the sentence in such a way it will make sense
Know that a sentence is structured Subject⇒Verb⇒Object.
The Subject is what/who is carrying on the action.
The Verb is what is being done
The object is what the action is being done to.
I in this case is the subject.
Buy is the verb
toothbrush is the object
'Note in this case we also have some parts of speech included, "need" is an adverb of which is describing how bad the condition is, or how desperate you need the toothbrush. I will come before the verb.
"New" is an adjective that is describing what kind of toothbrush you need, since it is describing the the object it comes after the object.
I need to buy a new toothbrush is your sentence that makes sense.