Here are some tips on how to give (Math) Talks. Talks on other scientific topics should be similar.
One tip I found very useful is this: For long talks (1 hour) and above, it is better to use the whiteboard / blackboard. As this will give you “the flexibility to add or omit material as you see fit, and it forces you not to go to fast.”
For shorter talks, it is better to use slides. “A good rule of thumb: you should allow between 30 seconds and 1 minute per slide.” So if you are preparing for a 30 minute talk, around 30-60 slides would be ideal.
This is advice by the legendary Paul Halmos. His first advice is to “Make it simple, and you won’t go wrong.”