Step 1: BOTH of you negotiate and agree on terms. If you don't understand how it works, watch the TV show "Shark Tank" on Fridays around 8pm, and get an idea of how an investor determines the value of a business.
Step 2: Once you agree on terms, have an ATTORNEY draft a contract, and then let their own separate attorney review it and make revisions, then you both agree on the final contract.
To answer your questions...
If it is an investment, the investor would purchase a percentage of the overall company OR you can limit it just to the actual clothing itself. So if your company starts selling something besides clothing, it's not included in the agreement, but if the person buys ownership in the company, it applies to EVERYTHING the business does. The partnership would last until the person sells their percent of the company.
Honestly, what are you offering?
The person has the money, so what is your contribution? Do you have connections to suppliers? Are you a designer with a nice resume? Do you have connections to retailers? Why would someone partner with you?