on another forum someone correcting saying that a fresh rotc lt doesnt enlisted but be commissioned. what is the difference

Marine Officers:
http://usmilitary.about.com/cs/marinejoin/a/usmcofficer.htm

Marine Enlisted:
http://www.military.com/Recruiting/Content/0,13898,rec_...listed_usmc,,00.html

Basically, an officer is mid-high level management, has a degree, and in most cases can resign after 4 years of service, but can serve past that without renewing a contract. Enlisted are the "worker bees" and serve on a contract (usually 4-6 years) and have to renew (or choose not to renew) the contract every 4-6 years. They can not resign at any time, and must serve the full contract once they enter.

There's also an overview of all the commissioning programs in the top link as well.

And I think this link will pretty much tell you anything else about enlisting/commissioning:

http://usmilitary.about.com/od/marinejoin/

can an enlisted person ever switched to commissioned. like he enlists into the marines reserves, takes hes four years of college and rotc, could he switch over to be commissioned into the active marines as a lt?

If you are going to college and in ROTC why would you join the reserves as enlisted? You should be able to get an ROTC scholarship after your first year if you dont get one right out of the gate (ie right after high school). Commissoined is the way to go, you can actually resign your commission before your initial contrat is up, at least in the national guard you can.

Commisioned Officers can resign at any time. There may be penalties involved, but thats all spelled out long before a zero receives his/her commission.

im just trying to get as much experience as i can, is it possible to do what i said in the post above

I recommend speaking directly to a recruiter to get the real answer. Most of what you will get in here is opinions and hyperlinks pointing you to likely sources to the question.

Recruiters for the branch of service you are interested in, is the smart way to go. These folks live and breathe for these types of questions.

I would recommend talking to an officer recruiter or an ROTC recruiter or cadre, I would not recommend talking to a regualar recruiter.

There is no such thing as an officer recruiter, and ROTC recruiter is a myth.

ROTC recruiters belong to the local recruiting company, referred to as On-Campus Recruiters (OCR).

Officers are recruited by REGULAR RECRUITERS. Recruiters in all four branches are NCOs. Direct Commissioning programs still are run and managed by NCOs. There is no recruiting branch for officers to affiliate to in any service.

The first link I posted above describes almost every Marine Officer commissioning program, including those for going from enlisted to officer. Yes, it's possible to enlist, then get a commission later. Yes, going enlisted in the reserves and later getting a commission is also possible. As others have said-- see a recruiter for more information.