How do I determine my cost of capeital as an investor.?

I think we may be mixing terminology here. It's not ROI. Firms look at capital from either retained earnings (profits) borrowed money, or capital from investors. The "cost of capital" is what the ownership demands from its investment. This is viewed as a cost from the firms' point of view.
A lot of equity traders use the idea of "cost of capital" as if they were borrowing money to invest in operations-like using margin in a brokerage account.

Think about the term. "Cost of capital" means how much are you paying to use this money. So, if you borrow at 10%, and are gaining 11%...your cost of capital would be 10%.
And if you are gaining 2%, your cost of capital is..........still 10%.

Doesn't matter that you're getting skinned alive and losing 8%.

known as roi(return on investment)cost of capital is determined taking into account present lending rate and opportunites available as alternatives.if the lending rate is say 10% and alternative investment yields say 10.5% ideally a % midway btwn 10 and 10.5 can be considered as cost of capital.The above matrix are very volatile

The cost of capital can be calculated in many ways depending on industry. For an investor however (I'm assuming you are investing in the stock market or property), it is usually calculated by "risk free return" + "market risk premium" * "beta"

a. Risk free return is the return on an investment with no risk. This is essentially the return on government bonds.
b. Market risk premium is the return of the total market minus the risk free return.
c. beta is the market sensitivity of a stock.

Example: Suppose the government bond rate is 4%, the market return is 13%, and the beta of an investment is 1.3

Cost of capital would be 4 + (13 - 4)*1.3

By using this method, you are compensating the extra risk in the market (market risk premium) and the extra risk of an individual investment (beta). You can then use the cost of capital as a percentage to find present value etc.