First a warning: the top 3 countries for marriage fraud by women are Philippines, Thailand & Russia/Ukraine. In fact, Russia/Ukraine is run by Red Mafiya organized crime, used to bring in their criminal operatives. If you met online, especially on a "dating site," the probability of fraud exceeds 99%. Unless you two originally met in person, e.g. while attending university, steer clear, as you are highly likely being set up. Of course, if you met while attending university, there's still a high probability that she set out to trap a husband solely for immigration purposes.
Now, if you two have met face-to-face, spent time together in person, within the past 2 years, you can apply for a fiancé visa. You must prove you earn enough to support yourself, all your dependents, plus your fiancé, and have a home for all of you. If you have no dependents, just the 2 of you, you must earn over $19,387 at present.
If approved for fiancé visa, you must marry in the US within 90 days of her arrival, then file for her change of status for to obtain a conditional (temporary) green card. If the marriage continues in good stead, after 2 years, she can file for removal of conditions to obtain a permanent green card. Note that it is common for such spouses to leave you as soon as they get a permanent GC.
Complete details on fiancé and spousal (CR-1) visas at uscis.gov.