Great choice; there are tons of deals in Harlem, and you can get a lot for your money.
In addition, there are plenty of income-restricted properties--which allow you to find affordable properties and reasonable co-op boards.
I live in an HDFC Harlem co-op myself, and specialize in working with first-time buyers. I'm also deeply into the foreclosure market, which can take longer, but provide significant rewards. For instance, there is a 2-br foreclosure in my building in the mid-$200K range; some of these also qualify for Fannie Mae Mortgage Programs, which allow you to put as little as 3% down (although the co-op may require more).
And while condos may seem appealing because there's less red tape--and sometimes ARE a great option--the biggest downside with a condo is the substantial difference in closing costs. The same apartment, if it is a co-op, could cost you $2-3000 in closing costs--but might be $15-20K if it's a condo. This isn't always a deal breaker, but it should be taken into consideration.
If you have a good job, good credit, and a co-signer, you are in better shape than you may think.
Finally, the best thing about working with a real estate agent is that since the seller pays the commission (as per the listing agreement), the agent's services are free to the buyer. There is really no good reason not to have an agent doing the work and negotiating on your side, although some unscrupulous brokers might tell you otherwise. Personally, I don't like having one agent/broker conduct both sides of the transaction unless I know them VERY well; remember that the agent is contracted to the seller. They should act in good faith for both parties, but since their salary is commission-based, there can be a tendency to want to settle at a higher price. This doesn't always happen, but it's another reason why it's good to have someone objective on your side, knowing what information to relay and what to keep private, as your deal is negotiated.
If any of this sounds like something you would like to know more about, send me an email!
I'm a licensed agent with Prudential Douglas Elliman and have worked on both buyers' and sellers' sides of lots of transactions north of 96th St. I am also a resident and strong advocate of Harlem's many neighborhoods, especially for first-time buyers. And I have learned through experience a lot about guiding first-time buyers toward finding what's really important to them and what might be better reserved for their "wish list" once they build some equity and are ready to trade up to their second apartment. ;)
If you've already found someone, good luck with your search. Buying your first place can be intimidating, but in this market, it is absolutely brilliant to have made the decision to buy.