There is no definitive answer to this question, as it largely depends on the specific paper and journal requirements. However, as a general rule of thumb, acknowledgements should usually be placed towards the end of the paper, after the main body of text.
Some journals may have specific requirements for where acknowledgments should be placed. For example, the journal Nature requires that acknowledgments be placed "on the title page, before the start of the main text". Therefore, it is always best to check the specific journal requirements before submitting a paper.
In terms of what to include in acknowledgements, common items include thanking funding sources, thanking research participants, and thanking colleagues for helpful discussions. However, there is again no definitive answer, so it is best to tailor the acknowledgments section to the specific paper.
Here is a helpful link with more information on writing acknowledgments for a scientific paper: https://www.nature.com/scitable/blog/firstpersonplural/acknowledgements_in_scientific
