If you are a US citizen and your fiance is not a US citizen who stays outside the US and if you plan to get married in the US, you can obtain fiance visa for him/her.

K1 visa:
The K1 fiance(e) nonimmigrant visa entitles the visa applicant to enter the US to marry an American citizen.

K2 visa:
Unmarried children, under 21 years, of a K1 applicant derive K2 nonimmigrant visa status from the K1 parent if the children's names have been included in the K1 visa petition. A separate petition is not required if the children accompany or follow the K1 applicant within 1 year from the date the K1 applicant is issued his/her K1 visa. To be eligible for K2 visa status the child(ren) must be unmarried, eligible to be listed under the K1 applicant's visa classification and under the age of 21 at the time the child(ren) enter the US. After the child enters the US, the stepparent/stepchild relationship for US citizen petitioner and K2 child must be created before the child reaches the age of 18.

It is a multi-step procedure:

• Fiance visa petition
  File a fiance visa petition with USCIS on behalf on your alien fiance.
  
  
• USCIS approval and transfer - NVC
  Upon approval of fiance visa petition, USCIS transfers the case to NVC.
  National Visa Center (NVC) further processes your case and transfers it to the consulate in fiance's home country.
  
  
• Consulate interview
  Alien fiance and his/her children below 21 years of age go through consulate interview process.

In rare instances, some couples may have to attend a fraud interview if the government has any doubts whether the intended marriage is a real one.

The marriage must take place within 90 days of your fiance entering the US. If the marriage does not take place within 90 days or your fiance marries someone other than you (the petitioner), your fiance will be required to leave the US. Until the marriage takes place, fiance is nonimmigrant. It is not possible to extend the fiance visa beyond original 90 days duration.

It is best to get married as soon as possible after entering the US as getting marriage certificate takes time and it is required to apply for Adjustment of Status.

If you could not get married within 90 days for unexpected reasons and if you still want to get married, get married as soon as possible. However, your spouse would need to file Form I-130 along with Form I-485, Adjustment of Status application. Don't leave the United States if you are late by 6 months or more as you would be subject to 3 years (more than 180 days illegally) or 10 years (more than 1 year illegally) bars for re-entering the US.

If you are late in getting married and filing green card application, and USCIS catches you and puts you into removal proceedings, you will have to file the green card in immigration court. Application paperwork is same but the immigration judge will look harder at your case to make sure that the marriage is bona fide, and you had genuine reasons for delay.

After marriage:
After marriage, your fiance (now your spouse) can apply for Adjustment of Status to be able to live and work permanently in the US. If your fiance does not want to become a permanent resident after marriage, your fiance (now your spouse) must leave the US within the original 90 days duration.

Work permit:
K1/K2 visa holder can file the application for employment authorization.

If you file the the application immediately after arrival but before filing for Adjustment of Status (AOS), this EAD will expire when your K-1 visa expires within 90 days. However, because of the wait time to get EAD itself is more than 90 days. Therefore, it is a waste of time and money to apply before applying for Adjustment of Status. If you still want to apply, filing category is (a)(6).

You can apply for EAD at the same time as filing for AOS or after filing AOS. This procedure is same for all other applicants applying for AOS.

Social Security Number:
K1 visa holders can get social security number upon producing a valid unexpired I-94. Apply two weeks after arrival in the US so that Social Security Administration(SSA) can verify your information in the SAVE-ASVI database. If you apply too soon, they may have to do manual verification by filing Form G-845 with the USCIS which may add several weeks of delay. Also, do not wait too long as many SSA offices will not allow you to file for a SSN if your I-94 is about to expire within 14 days. Of course, it is also possible to apply after you get EAD.

Along with all other documents, carry RM 00203.500: Employment Authorization for Nonimmigrants just in case the staff at SSA is not aware that K1 visa holder is eligible to get SSN.

K2 visa holders must first have work permit (EAD) to get social security number.

Carry your marriage certificate to the social security office to get the social security card in the new name. If you applied for social security number before marriage, you can go back to social security office after marriage to get your name changed in the social security card.

Social security card will be marked "VALID FOR ONLY WITH USCIS AUTHORIZATION". That means, you need to get EAD first before you can actually work.

Travel outside the US:
Fiance visa (K1/K2) allows a fiance to enter the US only once. If you leave the US after entering on a fiance visa, you may not re-enter on the same visa and you will need a new visa.

After getting married and after filing Adjustment of Status, if you want to leave and re-enter the US, you must apply for Advance Parole to return to the US.