Always allow time for questions. If your talk has been as interesting to your audience as you hope, they will have some queries and these should not be dismissed. It does need careful planning to allow for this time and I admit that I am not the best at allowing enough time but I have learned some lessons through making many mistakes in the past.
You do not have to have questions at the end but it makes it easier to control, especially of you are running late. Questions as you go along are sometimes easier to deal with, because they relate precisely (or should do) to what you have just said; by the end, people might have forgotten what they wanted to ask and you might have forgotten exactly what you said.
You must decide which way you would prefer to do it and then stick to your plan. Make it plain at the beginning that you will accept questions but only at the end. Reiterate this politely if anyone interrupts you.
Questions as you go along can throw you and make you dither when trying to get back on track. If this is the first time you have given a talk, it might be best to leave time at the end, so that you can have greater control. However, most audiences prefer to ask as you go along and can be difficult to stop.
Try to anticipate questions. Read through your script or notes and work out what you haven’t dealt with in great detail and prepare in your mind to give some details if required. You cannot say everything about any subject in a short talk but planning for questions does allow your audience to choose which part of your subject they would like expanded.
If you really cannot think of the answer then do not try to bluff your way out. Someone will be able to catch you out, either there or afterwards and if you have not been truthful, you will not be asked back and will get a bad reputation. Once you have started to give talks to local groups or businesses, word gets around and you will be asked to speak to others, but not if they do not trust you. Admit your inability to answer but promise to find out and if any are really interested you could let them know. If you have sources you regularly refer to and can give those, such as books or websites, it is helpful and will often circumvent any lack in your knowledge.