Hi Paulina,
I wrote an article on warm-up activities, which you can find here. I also posted some information on warm ups in my weekly newsletter, which I've cut and pasted below.
The Importance of Warming Up Students | Heads Up English | ESL Lessons
In short, activities at the start of the lesson are very important for the following reasons.
1. They set the tone for the lesson.
2. They get students to begin thinking in and focusing on English.
3. They provide a transition into the topic.
4. They allow you important opportunities to assess ability and character.
The article, though, doesn't offer any ideas for effective warm ups. So here are a few tips to get you started with more activities at the start of your lessons.
Idea #1: Questions and Answers. Write two or three questions on the board, preferably related to the topic. However, for classes that meet regularly, you may also use this as an opportunity to include target language from the previous lesson. Read the questions aloud and check comprehension. Students then find a partner to discuss these questions for the length of the warm up. As no pair of students should finish talking early, this means that everyone will need to ask follow-up questions to generate a conversation.
Idea#2: How Many People Can You Talk To? Write two questions on the board. Students then find a partner to ask and answer each question. After both questions have been covered by both students, each finds a new partner. They then repeat the process. By talking to many people many times on the same topics, answers will show improved accuracy and fluency.
Idea #3: Speculation. This idea can be used with just about any grammar point. Although you may use a picture from a magazine or a website for students to talk about, I prefer to have students speculate about me or other teachers. For example, if the grammar point for the lesson were on the past tense, students could discuss and write ideas to answer the following question: "What did your teacher do this weekend?" Students can generate realistic ideas for your weekend, or even completely off the wall ones. This activity works well with many different grammar points.
Idea #4: Find Someone Who... Write five questions on the board, all of which are yes/no questions. Even better, if time allows, create three sets of handouts with five different yes/no questions on each handout. Students find a partner, ask/answer one question, and then find another partner. They want to find people who will answer "yes" to the questions, as the activity is called "Find Someone Who..." A "yes" means they can check off that question, and then ask other questions on the handout.
Idea #5: Charades. Write actions on slips of paper before the class. Place students in small groups and give each student one slip of paper. They must act out the action on the paper without speaking, and their group must guess the answer. You can focus on just vocabulary, so students will answer: "play soccer" or "eat" or "watch TV." You can plug the verb into a sentence, such as: "You (verb) last weekend, didn't you?" Continue for five or ten minutes so that everyone in the group has had a chance to act out the action.
If you have any questions, don't hesitate to ask.