I understand that it is difficult to place students in their levels, especially when you are a new teacher.
I suggest you start by finding out how your school has placed students in the past and try to get copies of tests. You can also look at the books used in your school. This will help you to see what the students study at each level. If you have to design your own test, start with easy questions and build up to more difficult ones. Base the questions at the beginning of the test on language that is studied in the beginner book; the next set of questions on the test should be based on the book used at level 2 (elementary or pre-intermediate), etc. The test can be oral or written.
If a student does well on the first part of the test but doesn’t succeed on the second part, then you should place that student in the second level and so on, for other levels. Remember that some structures, such as the present perfect, are first introduced in beginner books. However, it takes students a long time to master such a structure. If a student gets the present perfect wrong, it doesn’t mean they should study at beginner level.
Remember to spend some time speaking to the student to find out how much they understand orally and how well they speak. Again, the conversation should be based on topics and language studied in the various books used at your school. As a general rule, an intermediate student should be able to speak easily about their everyday activities, their work and opinions on common subjects.
As for the web, there are many placement tests on internet but you usually have to pay for them.