Basically it’s all about money. If you get married to her then apply for a visa for your spouse you need to have enough money in the bank and a good salary to show she will be fully supported by you. They also check to see if it’s a serious relationship not a sham one for a passport only.
So boxes to tick:
Married
Serious relationship (what colour are her eyes etc)
You have a house/flat.
You earn above £27k pa.
You should then get a 2.5 year visa. Costs a couple of grand for visa.
She has to stay for 10 years before you can apply for indefinite leave to remain. Then citizenship and UK passport.
All in all very expensive. Oh and the home office is very slow and inefficient so be prepared for frustration. You may need to use an immigration lawyer and invoke the ECHR. Although be wary the tories want to scrap it and create a UK bill of rights.