If she intends to reside here permanently then the most secure way to do this would be for you to marry here and then apply for a spouses visa after marriage.
You would have to apply for a certificate of approval for marriage prior to marring. There is information on applying for the certificate here:-
You will have to apply by using Form COA and you can download this from the UKBA website.
Once you are married you can apply for a spouses visa.
You will have to show:-
You will need to apply for settlement (ie. spouses visa) by using form FLR (M) Settlement, available for download from UKBA's website. It would be best for either a solicitor in the UK to prepare it in consultation with both you and your wife before submitting it.
Your wife can find Uk immigration solicitors through the following Law Society Website search engine:-
You could apply for an unmarried partner's visa but you would have to show that you have been in a relationship for two years prior to the application and less applications tend to get accepted under this method.
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