1. Keep your unit cost as low as possible. One-colour (and one-size) screen prints are the cheapest option for small orders of t-shirts and can still look really good if you get a good design.
2. Get good quality shirts! Ideally go and see the products before you get things printed on them, if you can get a good deal near where you are.
3. Girls want skinny-fit t-shirts! Having said that, you won't sell any Large ones...the larger girls tend to get men's t-shirts in a medium size.
Keep the cost to the consumer low and try to ensure you're making a decent margin on each item. This will mean you'll break even quicker and hopefully generate something to put back into the next run. Make sure that your merch is good quality and is something people will want to buy, otherwise...well, they won't buy it.
Are there any specific subjects you want info on with regard to merchandise?