To get an F-1 visa approved, you will need to demonstrate that you have sufficient funds to pay for the first year of study and that you have resources available to cover the rest of your educational program.

For an M-1 or J-1 visa, you will need to demonstrate that you have sufficient funds to pay for all tuition and living costs for your entire stay in the US.

The information you provide on the I-20 form (F-1) or IAP-66 form (J-1) will be scrutinized very carefully by both the foreign student advisor at the school and the INS. If you don't have the resources necessary for study in the US, you will not get a visa.

You should know where your money is coming from before you jump on a plane.
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Several schools require proof that you have enough money for the entire course of study even for an F-1 visa, because too many international students are forced to return home after only a year of study.

If your education will be sponsored by a US citizen (e.g. a relative), the relative will need to fill out a Form I-134 (Affidavit of Support). This form requires them to pay your expenses if you can't. A Form I-134 filed by someone who isn't a relative doesn't count as much on Form I-134 filed by a relative.

Proving Non-immigrant Intent