Hi
About your first question, if x is a normal random variable(0 mean and variance 1), if you want to make a another gaussian random variable with meam 0 and variance sigma2, you should make it from x by sigma*x
Added after 16 minutes:
Hi
About your 2nd question.
White noise is confusing, specially in simulations.
brmadhukar is right. Noise power = sigma2 for N(0,sigma2). But what is sigma2 ?
Be careful, power spectral density(PSD) of white noise is flat. so power of white noise is infinite. If some filters are used, power become finite. If PSD of white noise is flat with amp N0/2, then after filtering, its power become N0*B and B is bandwidth of filter. so sigma2 = N0*B.
If u dont like to work with bandwidth, just make a normal random variable and multiply it by sigma.
see MATLAB help about AWGN and SNR, it hase some good points.