There is a HI hostel in each of your cities. Hosteling International hostels tend to be better than Backpackers and the independent ones. http://www.hihostels.com/
I have not stayed in any of these 3 but I/we stayed in the large one in Chicago.
Being able to cook and store food in a hostel kitchen is a big advantage.
Get a membership card if you are going to be staying in HI hostels for more than a week.
Travel between the cities is easiest on Amtrak trains IMO.

There will be plenty of people traveling between these 3 cities in September.

My kids have always used Hostels.com (www.hostels.com) as a resource for researching hostels in Europe and it can also be used as a resource for hostels in the U.S. You might look at some of these in addition to the Hosteling International hostels.

Travel on Amtrak between NYC, Baltimore and D.C. is good, but it can be expensive. A cheaper alternative would be by bus. Some reliable bus lines are Bolt Bus and Mega Bus. These bus lines offer tickets as low as $1.00 if you book far enough in advance. (BEWARE of any of the so-called "Chinatown buses" which also go on these routes. They are not reliable and have had severe safety issues--including deadly accidents--in recent years.) Another way to get between Baltimore and D.C. are the MARC trains.

"able to give me advice on where to go and what to see"
Sarah_jay, what brings you to the U.S and specifically those cities? You must have some things in mind so let us know and we can fill in. with more detailed suggestions
Baltimore is not usually on the list of "must sees" especially for those who have not been to the U.S.

New York has more than 50 million visitors a year and almost all come away unscathed.

HI hostel in NYC is a good - and huge - one - with, I beleive some of their own tours and activites.

Train or low cost bus bwtween the cities is easy.

And NYC and DC have a ton of free things to see/do to make a budget go further.

Get a Let's Go guide and chcek out the Thorn Tree sectin of te Lonely Planet website for advice from other students on a budget.

(And I understand NYC and DC - but Baltimore is a bit of a mystery. Not a bad city - just not an obvious choice.)

Camden Yards for a baseball game is within walking distance of the Inner Harbor area of Baltimore. Next to the Inner Harbor is the National Aquarium and the old flagship US Constellation.
There are water taxis if you wanted to go to Fort McHenry.
The city you are missing from your list is Philadelphia.
30th Street station is close to lots of tourist attractions in Philadelphia.

Yes, but NYC and DC have 20 times as many thnigs to see - as well as top class baseball teams,

<<<but NYC and DC have 20 times as many thnigs to see>>>

I'd put that figure much higher! Baltimore is a nice town, worth a 1-day visit (especially if you want to go to a game at Camden Yards, whether the team is good or not), but I wouldn't put it high on my list for a first-timer.

Thank you for the comments and advice so far! I have family in Baltimore who I have never travelled to see! So that's the main reason for Baltimore, I have been to the USA twice before but both times was Florida.

I love photography, so anywhere with that's picturesque and has nice monuments will make me very happy!

Well then you will love both DC and NYC. Huge numbers of monuments, memorials, world famous architecture and sights. (Statue of Liberty, Washington Monument - simply dozens.)

To me, the most moving place I have ever been in the Lincoln Memorial - not just for what he did but because of all the other events related to the civil rights movement that occurred there.

With a 41/32 record the Orioles are ranked #2.
http://baltimore.orioles.mlb.com/schedule/sortable.jsp?c_id=bal
My daughter chose Baltimore to get married last month.

Sarah Jay,

I live in Baltimore, but since you're visiting family there, I presume you're also staying with them, and that they will have plenty of suggestions/advice on what to see and do in Baltimore, if they haven't already planned it out for you--so I won't make recommendations there, unless you'd like some.

I used to live and work in Washington DC. I can't recommend it highly enough as visually breathtaking and beautiful city. And it's good that you'll be there in September, after the summer tourist crowds have left. Many of the monuments and major government buildings, with white limestone facades, are simply stunning, especially in the early morning or late afternoon light. I highly recommend seeing the Library of Congress and the Supreme Court, even if you don't go inside (although they are beautiful inside as well). They and all of the federal government agencies have websites with information for visitors. The Metro subway system is very clean and safe. I'm sorry I'm not familiar with the hostel(s) in DC, but if you tell me where it (they) are, I can tell you about their neighborhood(s).

Dupont Circle is a great neighborhood for young people to hang out in--very lively with great shops, restaurants and bars and picturesque neighborhood streets. No "sights" there, but a great neighborhood to stroll around or sit in a cafe.

The National Mall is a huge grassy area spanning several blocks in downtown DC, and the Washington Monument, Jefferson Memorial, Lincoln Memorial are all situated around it (as are numerous museums). It is also a nice place to stroll, sit on a bench, have a picnic, etc. I agree that the Lincoln Memorial is quite moving, as is the Vietnam Veterans memorial and the relatively new FDR memorial--also in that area.

The East Wing of the National Gallery of Art is also visually stunning in its architecture (and if you like modern art, it has a wonderful collection).

All of the above places I've mentioned are easily accessible by the Metro. The Supreme Court and Library of Congress are in the Capitol Hill neighborhood--you can get off at any number of Metro stops in the area--Union Station and Capitol South come to mind. Dupont Circle has its own Metro stop. The Smithsonian and L'Enfant Plaza metro stops are good for the National Mall. The National Gallery of Art is near the Gallery Place metro.

All of the areas I've mentioned are very safe, and you should not be nervous about visiting them on your own. However, as with most all major U.S. cities, as a precautionary measure, even in these very safe areas, I would not walk around by myself when they are deserted--which is generally only very late at night/wee hours of the morning. The exception to this are the blocks around the Capitol, Supreme Court, and Library of Congress, which have a fair number of police foot patrols well into the night.

Have fun!