There are different ways to express future in English. The most common one is with "be going to".

This video shows you the form "be going to". Read the explanations and watch the other videos.

To express future in English we can use different forms:

WILL
- I will watch TV (tonight)
BE GOING TO
- I'm going to watch TV (tonight)
PRESENT CONTINUOUS
- I'm watching TV tonight

The most common one is with "be going to", so if you want to express future but you don't know which form to use, use "be going to" and you will probably be right, and if you are not right, you will be understood anyway.

Attention:
- I'm watching TV = present
- I'm watching TV tonight = future
- I'm going to watch TV = future

Colloquial variation of "be going to"
GONNA I'm gonna watch TV
This form is very common in conversation, especially in American English.

FORMS
affirmative I'm going to play tennis
negative I'm not going to play tennis
interrogative are you going to play tennis?