Hey Becks,
That's a pretty vague question. Do you mean orchestral strings (realistic) or synth strings. For synth strings there are loads of presets for the malstrom and subtractor to use and tweak, although this isn't really creating a string rather using a preset. If this is the case then you are asking us how to program a synthesizer,

i'll pass on that one
As far as orchestral strings go, you are pretty much limited to what is in the Orkester sound bank, but there are loads of refills out there. Again you not really "creating" a string, rather using a sample.
To create one buy yourself a violin and hire out a recording studio,