Either by train, airplane, or car.
It's best when you have your own car. But anyway to see most attraction you can walk because they are many of them in the center and/or very close to it. The public mean of transport are quite good and come usually in time. But in the summer it's too hot in most of them. So better by car. You can rent a car from a company which has its office close to the Continental Hotel and also the Bastion place.
Timisoara is close to Serbian and Hungarian border, within one hour driving. We travelled to Timisoara from Belgrade by train. It took about eight hours.