There is much to consider when choosing a sleeping bag:
Down or Synthetic - down sleeping bags have better warmth to weight ratio, so if lightweight is your thing, you will want to go with down. You want to take care that your down sleeping bag does not get wet, though, as this will cause the feathers to clump and then the sleeping bag will not perform. Synthetic sleeping bags tend to be bulkier and heavier but perform better in wet conditions and are cheaper. You should definitely consider a sleeping bag liner to make your bag last longer. It essentially acts as a bottom sheet for your bag, so after a trip you can wash the liner and hang out your sleeping bag in the sun to keep it fresh.
Temperature rating - you want to consider the temperatures of your adventure destination(s). The comfort temperature rating is a guideline for the temperature at which the sleeping bag performs the best, meanwhile the lower limit is the lowest temperature at which the sleeping bag can still keep you warm, but its performance is compromised. It is best to pay attention to the comfort temperature rating to ensure the sleeping bag will perform well and keep you cosy on your adventure. Additionally, you may want to consider a thermal liner for your bag to improve the performance of your bag.
Shape - rectangular, semi rectangular, mummy, and double are the main shapes available for sleeping bags. Rectangular and semi rectangular bags have the benefit of being fully unzipped so that it can be used as a comforter and also means that you can roll around inside the bag with room to stretch out. Mummy bags have a snug fit, boosting warmth, and you will tend to roll over with the bag rather than inside it. Double bags are made for two people to sleep in at the same time, good for couples (some rectangular/semi rectangular models can be zipped together to create a double bag).