Hi Papi -- Welcome to the forums and congrats on your very first credit card!!
Yes, your security deposit is your limit. So you have a $200 revolving credit card limit. What this means is that you can spend up to $200 before you must make a payment to release additional credit. With revolving credit, you have to two dates to be mindful of:
(1) Statement Closing Date: This is the last day of your monthly billing cycle.
(2) Payment Due Date: Generally 25 days after your statement closing date. You must pay at least your minimum payment due by this date or you will be considered late.
For example, say your current billing cycle is from February 24th - March 23rd. And your payment due date is April 20th. And your minimum payment due is $35.
Between February 24th and March 23rd, you charged $150 on your card. You must pay at least your minimum payment due on or before April 20th -- to avoid being charged interest, pay your entire balance due by your due date. ***Note -- if you only pay $35 minimum payment, then your remaning balance owed will be $115 and your available credit will be $85. Your available credit will not return to $200 until you pay your balance in full.
Your next billing cycle will be from March 24th - April 23rd, with a due date of May 20th. Rinse and repeat.
Now, if you charge $200 on your card before your billing cycle ends, and you want to keep using the card, you will need to make a mid-cycle payment on your account to release additional credit so you may continue to use your card.
You may increase your limit be adding funds to your security deposit. Citi secured cards do graduate to unsecured (I believe) after several months of responsible use. You can search the forums for posts about 'citi secured graduation'. Once your card is graduated, you will be refunded your security deposit and granted an unsecured line of credit, after which, your limit can grow further by requesting credit limit increases every six months.
I'm guessing you have no FICO scores yet because this is your first ever account (unless maybe you have student and / or auto loan...)? It takes 6 months of reporting to generate FICO scores. So once Citi reports your 6th statement to the bureaus, you should be able to pull your FICO scores from all three bureaus to see where you stand.
For future reference once you do start monitoring your scores (and you should monitor them when the time comes):
The other thing you must be cognizant of is utilization. Utilization is the ratio of credit used and your credit limit (balance / credit limit). Since you only have one card, it will be the only source FICO will be able to use to determine utilization. FICO will penalize you for high utilization and no utilization. High utilization is anything over 28.9% and no utilization is, obviously, 0% (ie. a $0 balance on your card). You want to ensure that when your card reports to the credit bureaus, it reports a balance of less than 28.9%. 28.9% on a $200 limit is $57.80.
Citi will report whatever the balance is on your Statement Closing Date. To ensure that a balance of less than $57.80 is reported to the bureaus, you must pay down your balance before this date. So, using the previous example: If your billing cycle is Feb. 24th - March 23rd, and you charge $150 between that time, then you would need to pay your balance down from $150 to an amount less than $57.80 before your statement closing date of March 23rd. Then, you would pay the remaining balance (the less than $57.80) off before your due date of April 20th.
The higher your limit, the less of a hassle utilization can be because you can allow higher balances to report without going over the recommended util. Also - utilization has no memory -- so if one month you forget or are unable to pay your balance down before the statement date, and a high balance is reported to the bureaus, any FICO scoring penalty you receive from high utilization will be reversed (points lost will be regained) once the account reports a lower balance the following month. So it's not the end of the world...
For now though -- just focus on paying your bill on time, everytime. Read through the forum posts to learn more about how credit works and different tips for obtaining and maintaining a good credit history. In six months, you'll be ready for your next steps... so be sure to post back with any questions you have or advise you may need 
Enjoy!
@Anonymous wrote:
Yes, that makes sense. Over time if I gain an unsecured credit card, what will this mean? How is it different than a secured card?
No deposit is required with unsecured cards. The creditor gives you a credit limit based on your creditworthiness (this is why your FICO scores and credit history are important).
Secured cards help people with no credit build a credit history because banks aren't really taking a risk -- your credit is secured by your own funds, which will be forfeited should you fail to pay what it is owed (the bank will keep your deposit to cover your debt). If you are responsible with a secured card, then banks will be willing to take the risk of granting you unsecured credit. With unsecured credit, they are taking a risk with their money, relying on you to pay them back. Unsecured credit for those with little credit history may start out with lower limits of $500 - $1500 but over time, as you build a positive, longer payment history, those limits will increase -- and you will be able to be approved for higher limits with other creditors (so long as your income supports it). Use your card, be responsible, make your payments.
