First, you'll need fill out your incorporating articles with your state's Secretary of State division here:
http://www.sec.state.ma.us/cor/coridx.htm
Under the "business filings" column, choose to create a new entity and follow the directions.
Keep in mind, that you'll also need to log in with your state's SOS division to file your annual reports.
After completing that, you'll have to register with your state's department of revenue and obtain a TID (tax ID):
http://www.mass.gov/?pageID=dorconstituent&L=2&L0=Home&L1=Businesses&sid=Ador
You'll also more than likely have to come back here on a monthly basis and file your AR-80 (or equivalent).
After filing with your state's business entity division and the dept of revenue, you'll need to register with the IRS and obtain an TIN (tax ID number):
http://www.irs.gov/businesses/small/international/article/0,,id=96696,00.html
I won't get into it here, but you'll have to complete your tax requirements as they determine you need to, more than likely it will be annually, just as you do your personal income tax.
The IRS is also your resource of W-4 information when hiring employees and W2s for wage earnings that you'll have to disburse by January 31st each year.
I can't stress enough the importance of keeping ALL of your documents and paperwork together and safe. Practice good records keeping and accounting from the start or else it'll come back to bite you in the ---.