Pilot training in a universityWatch
If you have interest in the RAF, you could join a UAS during your degree. However, you don't need any prior flying training to be a pilot in the Armed Forces; they will teach you everything and don't want to be having to get you out of any bad habits you have from any lessons. Better off spending time at uni developing your leadership potential.
I want become a commercial pilot, my sister said it's best do it from RAF and then join from there but I don't know how RAF works and how long service will last, also might not fit for it
Yes, people do go on from the RAF to become commercial pilots, but it's not a guaranteed way of doing it and not the reason they join up because, as you say, there are too many uncertainties, and it's not the way to go if you're not committed to the military lifestyle in the first place.
Please bear in mind, that the RAF require you to stay within the force for a MINIMUM of 6-12 years before you are able to leave; if you really want to become an airline pilot, I can't understand why people join the RAF just to end up leaving for a completely different goal.
Yes, being the RAF may help you when it comes to applications for airlines and flight schools - BUT, this is not guarenteed at all and a person who has never even touched the RAF has an equally good chance of getting into a school/airline.
Please think long and hard before joining the RAF just to become an airline pilot, in my opinion, unless you have another passion for air force helicopters or fighters, this can end-up being a poor decision.