I have recently started an MRes in Geography and I cannot relax because I cannot find out a basic timeline of where I should be at and when i.e. when my introductory chapter should be finished and I start my data collection. Also, I cannot find any information on writing an MRes thesis, how is it different from an undergraduate dissertation, should I structure it like a PhD thesis but shorter?

I would ask your department/supervisor if they have any advice or if there any rules. Usually in the student handbook or online course notes will give you a basic understanding of what is required of you. Though I would say it is safe to stick with the generic thesis template but slimmed down.

With regards to timelines, work backwards from your submission deadline and figure out what you need to do. Your first timeline does not need to be perfect but once you have an idea of what you need to achieve you can work out when to schedule it. For example, if you have field work your timeline might be categorised as before fieldwork and after. I would say writing is less important at the start as it is easier to write when you have everything done and it is better to focus on actually getting data. Though I would consider talking with your supervisor about all this as they might already have a rough idea of how low everything takes in your specific field.

PS: if you need ethics approval, drop everything and do that now! As the ethics process takes an ungodly amount of time and you want to get it in ASAP.