Before you analyse pieces of text you need to preprocess given text by striping punctuation, repair language, split spaces,lower the whole text and store the words in an iterable data structure.
For some basic sentiment analysis, following techniques can be used:
Bag of words
In bag of words technique we basically go through a bag(file) of words and check if the iterable made by us contains these. If it does then we assign some value to each word's presence in order to weigh the total sentiment of the text.
This link should help you understand more about this
https://en.wikipedia.org/wiki/Bag-of-words_model
Keyword Extraction and Tagging
Keywords and important information can be extracted from the input text by tagging the elements and then removing unwanted data.
For example:
My name is John.
Here John, name are the information and "is" isn't really needed.
Similarly verbs and other unimportant things can be removed in order to retain only the main information.
Chunking and Chinking helps.
This link must be of help.
http://nltk.org/book/ch07.html