Without other information as the other poster asked for:
How does you business plan on making money? Are you selling a product or service? If so, what price will you sell it for and how much does it cost you to provide the service or make the product. The difference between the two in your gross profit. Then, you have to understand your overhead - your fixed costs - like selling and marketing, salaries, rent, utilities, equipment (office equipment and equipment to provide your product or service), etc, etc. Any thing that is left over, after you pay taxes on that amount, can be either used to pay back your investor or to reinvest in the business.
If your business is web based and is designed to only provide information, how do you plan to make money from it? Will you use affiliate links, sell your information, or have advertising? Again, the difference between what you expect to bring in and what it costs you to provide these services is what could be used to pay your investor.
Regarding your second question, your investor should be able to determine how much of your business they will need to realize the gains they require on their investment. This will also be based on the first question. If you can say that your business will bleed off $1,000 a month in profits, then your investor should be able to determine the amount of the company they want to own based on portion of that profit they will get.
Example, let's say your investor wants a 10% return on their money in 24 months. To achieve this, the investor would get $553 per month for 24 months. That will provide the return of the investment and a 10% return on the capital. Thus, your investor would seek 55% ($553 / $1,000) of the company for 24 months. Now, if the investor has a longer investment range (more than 24 months), then the investor would have to determine a present value of the business - lots of methods to do that - then, the presentage of that value their investor would cover. Example, your business has a present value of $100,000 (this is perpetual value), the percentage of ownership would be 5% ($5,000 / $100,000).
Hope this helps