If it is swollen and itchy you will need to do something to remedy it and perhaps even consult a medical professional. This will give you peace of mind about the severity of the bite and let you know what to expect.
What Should I Do First?
First thing to do after you have determined what the bite looks like and the type of animal that may have bitten you, is to look at the discoloration of the skin. Compare the color to the pictures in the book to get a better idea of what it is. It may be red, white or even purple. Look through the book after you have taken note of the swelling and see if you can find a coloring and swelling that is the same color and shape as your bite. This will help you to immensely narrow down results.
Now take note of the bugs that could leave you with such a bite, then you will see where they are situated. Narrow down the bugs or spiders from your region and you should have a very narrow amount and will more than likely know the type that bit you, how severe it is and what to do to remedy it.
There are many bites and to be honest it is highly unlikely in most cases they lead to anything but a bit of itching at worst. However, make sure to contact a doctor if you have any doubts, as the old mantra goes, it is better to be safe than sorry.