How do I determine the start and end of the wave?
Also can you confirm whether a wave form can contain more than one frequency?


Yes. This is the basis of Fourier analysis. Any periodic signal (finite-powered) can be decomposed into a sum of sinusoids. We conversationally say the signal has content or power at all of the frequencies of the sinusoids needed to construct the signal.


  My problem is once I found the highest amplitude, how do I determine the start and end of the wave


If you are trying to find the period (or frequency) of the signal, you are asking about spectral estimation.

A very simple technique is not to find the "start" and "end" of the wave, but rather to find the zero crossings (after removing any dc components). Since each period of the signal will have two zero crossings, you can count some (odd) number \$N\$ of crossings at times \$T_1...T_N\$, and get the average period over that sample as
$$\frac{2(T_N-T_1)}{N-1}$$
Depending how precisely you need to estimate the period (and what sampling rate you sampled the signal at) you may want to use some interpolation between the samples to get good estimates of the zero-crossing times.

A (slightly) more sophisticated method is to make a periodogram. This essentially means taking the Fourier transform of the signal (by some numerical method, such as the FFT), and looking for peaks in the spectrum. If the signal is well behaved, the lowest-frequency (non-dc) peak will correspond to the fundamental frequency of the signal. 

The periodogram is actually not a particularly good method to find the fundamental frequency, but numerous more sophisticated methods (mentioned in the Wiki article) refine it to improve the ability to estimate the signal frequency.