The first step towards a thesis is finding a supervisor and determining an actual thesis topic.
(Note that all advise is personal, and your own supervisor may disagree)
Intended for: BSc, MSc, PhD
1. Determine which topic you are interested in.
Take the course: To start a bachelor/master thesis, it's strongly advised that you have followed the course(s) on the particular topic, and passed them with a good grade. Otherwise, it is almost impossible to write a thesis, since you will need most of your time to study the topic.
Follow the seminar: For a master's thesis, there are often advanced courses / seminars that specifically prepare you for a thesis in that field, since they discuss actual research papers.
2. Find a supervisor.
Contact the teacher: Once you know your topic of interest, contact a potential supervisor within your university. You can for example start with the teacher of the particular course.
Let them help you on: They may not always have time (many students may approach them). In that case, ask them if they can help you find an alternative option within the university (the research group on the topic is usually much bigger than just the specific teacher).
3. Determine a thesis direction.
Blend: There are two options here: either you come up with an idea, or your supervisor suggests a topic. Often it will be a blend: your supervisor suggests a direction, and you read up and gradually build upon it, or you have an idea, and your supervisor molds it into a feasible research plan.
4. Write three small thesis proposals.
Three proposals: Once a high-level thesis direction has been established, I advice to write three small thesis proposals (even if you already have a very clear idea).
Force yourself to read related literature for ~2 weeks and come up with three different ideas. This way, you prevent that you zoom in on an idea too early.
This phase is often skipped, but actually very important. You will work on your thesis for quite a while, and you want your plan to be solid (a few weeks is actually not much at all for a solid idea). Note that the time spend on reading up already counts as thesis time.
Template with bullet points: For each proposal, I advise the 'bullet point' approach of this template document.
5. Decide on a final thesis proposal & supervision plan.
Together with your supervisor, agree on:
Final thesis direction. Discuss your thesis proposals and agree on a final thesis direction. Work this project out into a full proposal (which mostly means detailing the experiment plan and adding a timeline).
Supervision meeting plan: You can either plan fixed meetings (in a certain regular interval), or ad-hoc meetings (you contact your supervisor when you want to meet). Be sure to send in all your progress documents before every meeting (see Templates).