Confused about taxes and being married....?

me and my girlfriend of 3 years want to get married, but i heard being married affects your income tax return. i would like to know in what way will it affect our taxes? i make 35-40k annually and my girlfriend 12-15k. i have one child with a diffrent girl and ive always claimed him as a dependent. for the last 3 years my tax return has been 4-5,000. my gf gets 800-1200 the last 3 years. if we do our taxes as married status, (jointly) will our tax return be less? or what diffrence will it make? also, there is that other option, being married and filling separate, if we do it that way, will it be the same thing as filling single? or how does that work? im just really confused about all this and id really appreciate it if someone can break it down for me, and explain this to me.. thanks!!!!!