1. Tom Yum soup can be easily made using water. But for flavor pack broth, it's best to use prawn stock. Chicken broth can be used too but it will obviously have chicken flavor which some may not like in their tom yum soup. To make prawn stock, you'll need heads and shells of 2 lb prawn and 4 cups + 2 tbs water. Fry the prawn shells and heads (with some oil) in a stockpot until fragrant and pink, then add the water and bring to boil. Simmer covered on low for 30 - 45 minutes, strain and use as stock. Make a large batch and freeze for future consumption.
2. How much chilies you add and whether or not your deseed all or half the chilies will be dependent on how spicy you like your soup. If you're not sure, deseed all the chilies and add the chilies first. If you want more heat, you can always add the seeds into the soup.
3. To reduce total cook time, I usually prep the ingredients for the stock first (lemongrass, galangal, kaffir lime leaves and chilies). While the stock is cooking, I prep the rest of the ingredients.
4. You can replace or add other ingredients eg. squid, claims, other greens or mushrooms.
5. I use rice vermicelli because that was what I had on hand but you can use whatever noodles you like. I cook the noodles in water first because they release starch which if cooked in the broth would thicken the broth.
6. Lemongrass, galangal and kaffir lime leaves can be found in Thai markets or some Chinese markets. Depending on your grocery store, you may find Thai roasted chili paste at the International aisle (Thai Kitchen brand can be found at Whole Foods).