Report to your local courthouse and ask for an affidavit form. Affidavit forms are readily available for free at most local courthouses, or you can draft your own on a word processor. Have your affiant recite the facts to you or write them out, and if they agree, they can sign before a notary.
An affidavit is a legal document that contains sworn facts and statements. Most of the evidence in any lawsuit is derived from the information contained in an affidavit. Since the affidavit is written and sworn to under penalty of perjury, the information contained in that document is just as effective as information given from a live witness at a trial. You can get an affidavit simply by drafting one and having the affiant (the person who is signing it) sign it before a notary public.
TL;DR (Too Long; Didn't Read)
To obtain an affidavit, write out the statements to which you want someone to swear in affidavit format. Have the person review the statements and, if they're true and accurate, that person should sign the affidavit before a notary public. Affidavits are executed under penalty of perjury.
How to Prepare an Affidavit
To start, report to your local courthouse and ask for an affidavit form. Affidavit forms are readily available for free at most local courthouses. The forms may be filled out by following simple instructions provided on the form. A form affidavit is not required, however, for a valid affidavit. All you really need are the caption of the case (if any) and a statement at the top that says the affiant swears or affirms that the following statements are true and correct to the best of her information, knowledge and belief. This is followed by a statement or a numbered list reciting all the facts she is swearing are true. It should end with a statement that she swears or affirms under penalty of perjury that the statements in the affidavit are true and correct to the best of her knowledge, information and belief. She should sign below that, in front of a notary public, who will then notarize the affidavit.
An affidavit can be prepared from the form or from scratch on your computer.
Going Before a Notary Public
Generally, affidavits must be notarized. Many banks offer free notary services, as well as some libraries. The notary will require that you or the witness sign and date the document in the notary's presence. The notary will then act as a witness of this and affix the notary's seal of commission. Once signed and notarized, the document is an affidavit. In some states, attorneys may notarize documents.
The Effect of an Affidavit
Affidavits are often used in court proceedings to support statements made in motions and petitions. The evidentiary effect of an affidavit can be tricky, however, because it is technically hearsay, which is an out-of-court statement presented for the purpose of proving the truth of the matter asserted. If you present an affidavit as evidence, it will need to withstand a hearsay challenge under a hearsay exception. In federal court, hearsay exceptions include things like public records and excited utterances. However, an affidavit may end up being admissible under the residual hearsay exception in federal court that gives the judge some leeway as long as you give your opponent reasonable notice of your intent to offer the affidavit and what's in it. State laws may vary, however, so if you're not in federal court, check the evidence rules for hearsay exceptions.