It appears you have been charged with an IRC 7201 violation, which is the crime of willfully attempting to avoid paying your taxes. As explained more fully in another Q&A, one of the things the government must prove beyond a reasonable doubt is that you acted willfully. It will try to prove this directly (e.g. with any written statements you may have made) or indirectly, through “Spies-type” conduct—circumstantial evidence like making a false invoice, making fictitious accounting entries, maintaining a double set of accounting records, and the like.
You must show that you did not act willfully (or, at least that the government’s evidence that you did is no good). The evidence you will want to present depends upon the sort of Spies-type conduct the government points to. For example, suppose the government maintains that you have been underreporting your income, and engaged in a pattern of tax understatement. You could reply that you did not act willful because you relied on a competent tax advisor, or you were somehow ignorant of the law, or you mistook the law. Unfortunately, such defenses are often insufficient because the mere claim that you relied upon another, were ignorant, or made a mistake would make it “too easy” for you to “get off the hook,” as it were. Qualified legal counsel can help you craft the best defense for your particular circumstances for the specific Spies-type conduct that IRS assets you engaged in.