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With €800-900/month you are near (but above) the official poverty  line - so it is possible to survive, but you cannot expect any luxuries. Depending on where you study, you'll spend half of what you have on rent (for a simple sublet room).
Students are allowed to work part-time on the side. But study courses are designed to be full-time, so expect your results to suffer. The minimum wage of around €9/hour is not universally enforced in the kind of McJob you are most likely to find.

To further what Beppi wrote, the way some students survive on such an amount usually depends on either having a cheap room through the university student services directly or a room in a shared apartment known as a WG in Germany. This might get accommodation down to just 300 – 400 Euros a month. Then if one cooks their own basic food from a supermarket and/or eats at the university student cafeteria (Mensa) then one can just survive. But then eating out, entertainment etc. have to be curtailed or financed through some additional income and most part time work available to students is going to be for low pay. Many people seem to be a bit unrealistic about how disciplined one needs to be to stick to such a tight budget. It not only means no luxury but actually finding things at special prices. Say one needs new cloths. There are shops like C&A in Germany that have decent stuff for cheap prices. And then they occasionally have sales where the normal prices might be 50% less. A good pair of pants for example on sale for 15 Euros instead of the normal price of 35 Euros, which at a normal department store would cost 70 Euros and at an upscale shop 120 Euros.

Thank you both for your replies/information. Another couple of questions, pls. So in your opinion, what would be the best budget if say I wanna go out every weekend to a club/bar/cinema.. Etc, not just surviving (you may take the accommodation out of the budget)? And about the part time jobs, is your limit 20 hours a week? And is it realistic to work 20 hours?

P.s. Berlin is where I'm planning  to study.

If you want to go out and it depends on your taste I think €200 minimum could be added to your budget.

But if you are going to work part-time, I'm not sure you would have time to study got to school and party.

It is very difficult to figure out the budget YOU need, without knowing all YOUR expectations and spending choices.
If you want to go out once every weekend, SimCity’s €200/month will only allow you limited choices (i.e. cinema and restaurant meal, OR a couple of drinks at a bar - but not both!) if you want all of it at some fashionable night spot, better budget €800-1000/month for the weekends alone (double that if you invite a girl-friend).
But since you mainly come to study and only work on the side (a €450 job is the best choice then, as above that formal requirements and costs - taxes and insurances - kick in), Sim is right to point out that you will have neither time nor money.
From a financial standpoint, it makes more sense to focus on good grades and get a better paid job after graduation.

is 800 euros enough monthly for food, etc (not including rent and university) please let me know, as I am coming from Hong Kong!

If you can live without luxuries like non-self-cooled meals, home country food, free time activities and travel, etc., then it is sufficient if rent and school fees are covered from other sources.

Apart from food (10-15€ per day), you’ll need transport (monthly ticket depending on location and distance around 70€), health insurance (approx. 90€/month, more for postgraduates), mobile phone and Internet (approx. 40€/month). The rest should (barely) cover toiletries, clothing, books, etc.