The best place to start is to read the general guides that your institution offers.

If you are a student from the University of the Aegean then go here :: Διπλωματική Εργασία, Κανονισμός της διπλωματικής εργασίας. 
If you are a student from the Hellenic Open University follow these guides :: Διπλωματική Εργασία

Always refer back to these guides as they contain all the information and rules that your thesis should comply with. 

A thesis is often a first step in a career path and thus it should be taken very seriously.

Chose a topic that is within your research interests and background. Never try to do something fancy, just because it is a trend, or because others told you to do so. Consider carefuly in which disciplines you are strong and have a good knowledge and then do some serious research on the current “state of the art”. Research moves forward very fast and you should always be aware of its current state. Do desktop research on papers, books and other theses on the subject you are interested in. 

Never ask your supervisor to decide for the topic alone. Never go to a meeting for discussing your thesis withought first to have a solid idea about the area.

Steps to follow in order to choose a topic:

1. Check requirements from the institution.
2. Decide what type of research you want to do:
   1. Existing literature review,
   2. carry out practical research yourself in the form of field research or experimental research,
   3. theory or methodology.
3. Select a global research discipline and focus in understanding the research areas that it can be expaded.
4. Within the research discipline, look for current articles and papers. Select at least a couple of articles from these top journals that present issues that fal within your area of research. I personally start from: http://scholar.google.com
5. Select a minor detail or characteristic within the research discipline and explain it in few words.
6. Create a hierarchical list of topics that fall within your interests. Try to keep it sort.
7. For each topic try to find at least 3 to 5 papers that deal with some research problem related to your idea.

There is a number of good sources that can assist you in understanding the research area. Those are Conference Papers, Journal Papers and Books. Of course you can use websites, blogs, online libraries and other digital and analogue sources but be alerted that not everything is considered a valuable source, especially those that are not peer reviewed. There are even fake journals and conferences not to mention websites and other online sources. Be suspicious and careful about your choices.

Best search engines are Google Scholar, Plos One, Sciencedirect, ACM Digital Library. Here is a list of search engines that I will update from time to time:

Google Scholar (http://scholar.google.com/)

ScienceDirect (https://www.sciencedirect.com/)

CiteSeer^x (http://citeseerx.ist.psu.edu)

ACM Digital Library (https://dl.acm.org/)

PLOS ONE (http://www.plosone.org/)

Directory of Open Access Journals (http://www.doaj.org/)

GetCITED (http://www.getcited.org/)

There are two different groups here, those who study remotely (ΕΑΠ) and those who study in campus (Aegean).

Students from the first group are intentionally studying from distance and have a limited number of interactions with their supervisors. Doing experimental work is often difficult and working in the lab impossible. Therefore most of the theses I supervise here are not involved in experiments and depended on lab equipment and software. Students should work with the means of hardware and software available to them (either by profession, or owned, or use open source), or do theoretical work (dissertation, no lab related experiments etc)

Many Aegean students also ask to do their thesis from distance. In my opinion it is possible, but there are many risks. From my experience most students that leave the university before completing their thesis delay their graduation significantly. There is a number of reasons for this: 

1. Most of the projects we do need lab work and are related to experiments that involve users and hardware/software equipment we only have in the university.
2. Based on this fact it is impossible to communicate and most importantly collaborate with the supervisor.  Students miss the potential of everyday interaction with the supervisor while email communication or other means of online collaboration are not mature technologies for these purposes.
3. It is hard to participate to hands on lab work, perform experiments, design and do evaluation with actual users.
4. Most students have difficulties in focusing to their primary task. They usually end up doing other things in life, work, experiment endlessly with various ideas irrelevant to their thesis, even take a year off and go on holidays!

All these are true time wasters in terms of thesis writing. In my honest opinion it is much better to stay focused, locate yourself in the campus and finish early. Then you have the freedom to do whatever you want!

This depends on a number of factors that mostly depend on the time and effort the student is ready to devote. The most important are:

1. Thesis is a full-time job, not part-time. Students that do other things in parallel delay significantly to finish.
2. Have a strong background on the research topic
3. Have a good knowledge in terms of methods, methodologies and techniques involved
4. Have a good understanding of how to perform research work
5. Plan early, follow the schedule and deliver on time the work agreed with the supervisor
6. Keep a good log of the tasks you completed and the tasks to be done
7. Do not waste time meeting with the supervisor without having something finished and prepared to present
8. After finishing up your work and writing you should hand in your final draft to the supervising committee. This process will take more than 10 days to be examined and give you feedback.
9. Students should take in serious consideration the reviewing comments and try to answer all of them.
10. Preparing the presentation will normally take a couple of days.
11. Handing in the final thesis to the system needs to be done 7-10 days in advance and according to the deadline given by the university.
12. Students need to present their work in public during the scheduled, thesis presentation days.

There is no general rule about the length of a thesis. It depends on a number of factors including the type of research work, the topic, the supervisor’s guidelines. In general we have:

Students from the Hellenic Open University should follow the specific guidelines given here: Guides EAP

The University of the Aegean does not provide a specific set of guidelines for writing a thesis. Therefore students are free to choose their own template. For those interested, based on the aforementioned format, I compiled a simple set of instructions that might help you save time in deciding the format of your text. Here is the template: ΠΑ-ΤΜΣΠΣ-Πρότυπο-Συγγραφής-ΔΕ_Σεπτέμβριος-2018