You own 90% of what? We don't know what the value of your company is, so first, you would need to put a valuation on it. There are many ways to value it, but you need to come up with something reasonable.
Then you have to find a person who is wiling to buy the 10% that you would like to sell. After that, it is merely negotiations.

I would approach this as you would pitching the company to any investor. You need to show how your company plans to grow, projected growth numbers, and your exit strategy.

If you don't have an exit strategy, then you won't be able to find an investor, and you will have to raise the cash through other means, such as a loan.

In short, we need more information about your company to make a short list of options and which are the most likely ways of accomplishing your goal.