1) You need to file USCIS Form I-864 - Affidavit of Support, and find a co-sponsor. The form I-864 is a contractual affidavit of support, and must be submitted for most family-based applicants and employment-based applicants when a relative is the petitioner. Here the co-sponsor can be anyone you know, it can be your relative or even a friend. (also see: www.greencardfamily.com/news/news2005/news2005_1221.htm)
2) Since you entered the country legally and overstayed, you can adjust your status in US after your marriage with a US citizen. But depends on when you entered the country with inspection and why you overstayed your visa, there may be additional issues that would complicate your status adjustment, so it may take more than a year to get the conditional green card.
Victor
www.greencardapply.com
www.greencardfamily.com