i want to buy a house with a fiance.i pay 90% of the house costs. what can i do to protect myself at signign so she has no legal right to half?

If you want to keep the house as your separate property, you will have to finance the property and take title only in your own name, and close the deal before the wedding.  To make sure that this is enough, you should check with a divorce lawyer in your area, because I'm not an Illinois lawyer and there is some variation in the law on this subject, from one state to another.

In most states, the property that gets divided in the event of divorce includes anything purchased from the wedding date onward, even if only one party's name is on the deed.  At the same time, even if you're not married when the title closes, if her name is on the deed and you get married after that, the property will still be "marital."  In some states, there are very limited situations in which a house bought in one name can be marital property, and you should explore those, if applicable, with your Illinois lawyer.

Your safest choice is to contact a local attorney and draft a contract similar to a prenuptial agreement. You can agree in the contract that you are entitled to 90% of ownership rights she is entitled to 10% or however you word it. You will have to be able to provide a very solid money trail that it is being paid by you etc. This can allow you to simply each be entitled to the percent ownership of your contribution.

She will have to agree and since you are getting married it can be a prenuptial agrement. Although you can never guarantee a court will follow the provisions of the agreement it may your your best option to protect your ownership rights.