Use "loose" to mean the opposite of tight or compact. It can also describe a sense of freedom. For example, "My clothing is starting to feel more loose with each passing week of my diet." Or, "The items were packed very loosely, and ended up getting damaged." Do not use the word "loose" to describe the reduction of weight. Even though it is often used incorrectly in this way, it is simply wrong in all cases. You go on a diet to "lose weight," not to "loose weight."
Use "lose" (past tense "lost") to describe no longer possessing something in any sense of the word. You can also use it as the opposite of win, or the opposite of gain. Some examples are, "The woman lost her husband in the war," "The team lost the game" and "I hope I don't lose my marbles before I even reach 50." You can also say, "I am trying to lose weight."
Sound out the word you want to use in your sentence. If the word you want has a "Z" sound in it, you mean to write "lose." If the word has a long "S" sound, you would write "loose."
Try using both words in a sentence together to better understand the differences. For example, you might write, "If I lose weight too rapidly, it may result in my having loose skin." The two words are not interchangeable. Students and anyone who does business writing should take careful note of the difference between these two words, which look and sound very much alike. If you don't, you readers may "lose" their patience with you, and your pockets will no longer be filled with "loose" change.