Multiply the time of the "certain point" by the sampling frequency, and add 1, in order to get the sample index in the signal. Access the signal at that index to get the amplitude there.
In order for there to be a "frequency" there has to be a change in value. At any given time, there is no change in value, just the one amplitude. Therefore you cannot have a frequency at any given point in time.
To be able to resolve a frequency of n Hz, you need at least max(2*n, 2 * Fs/n) samples.
Generally what you need to do is a short-time fourier transform (stft) such as is implemented by spectrogram or by https://www.mathworks.com/matlabcentral/fileexchange/45197-short-time-fourier-transformation--stft--with-matlab-implementation
