That's a big question! Basically, we use the simple form of the verb (e.g., "walk," "eat," etc.) when we talk about things we do all the time ("I walk to work every morning" / "I eat rice twice a week"). We use the past tense for an action in the past, usually by adding "-ed" to the end of the verb (e.g., "I walked 6 km. last Sunday"). However, there are some verbs (called "irregular verbs") that have a different word for the past tense (e.g., "I ate chicken last night"). You have to memorize these, I'm afraid. For the future, add "will" to the simple form of the verb (e.g., "I will walk to the store tomorrow" / "I will eat at a restaurant next Friday night"). There are other tenses (passive, perfect, continuous, etc.) but these basic rules are the most important ones.

This is an enitre chapter in English by itself and is the essence of Grammar. For basics, you must understand that TENSE tells us WHEN (The time at which) does the action take place. Sometimes it gives more information about what has happened (is happening or will happen) at the time. I recommend you to go through this website first. This will be a more comprehensive answer than what I can give here : http://www.englishleap.com/grammar/tenses and http://www.ego4u.com/en/cram-up/grammar/tenses