Oh my goodness! You’ve been blessed with the love of a kitten! May nothing ever tear you two asunder!
First things first, take your new fuzzy companion to the vet to get her checked out. The vet will make sure she’s got all the vaccinations and such she needs to stay healthy, and will get you on a check-up schedule. They’ll also help you schedule a time to get her spayed when she’s old enough (it’s for her own health, safety, and comfort don’t @ me). The vet will also have information for you on what to feed her, when, and how much. As she’s so little and no longer has her mother, this might be more complicated than with an older kitten.
Next I want to send you over to @drferox, an actual Tumblr vet who has pages and pages of advice for new pet owners. I believe they also answer specific questions.
Your kitten is probably suffering some abandonment anxiety right now. Make sure to give her plenty of cuddles and get her lots of socialization with you and other (vaccinated and friendly) animals. But be careful, as not all adult animals take kindly to baby animals and their instinct might either be to nurture or attack her.
Good luck, henny! You’re doing a good thing by adopting this tiny ball of needles and fluff!
2-3 weeks is very young, kittens have only just opened their eyes but may still need manual stimulation to urinate and defecate. You need to get into a vet clinic within 24 hours to check age, and be shown how to feed and toilet properly. Being a runt may or may not be due to anything medical, some runts will catch up just fine with lots of food, but you will need a kitten-appropriate milk replacer, which most vet clinics will have.
They are usually not anxious at that age, just wanting food, toileting and soft, warm things to snuggle up to, but your first stop is to check in with a vet clinic to make sure you’ve got everything you need, and you’re doing it right. Sometimes nurses will be quite happy to walk you through it before you even need to see a vet, but go today because 24 hours is a long time for a baby kitten.
Get to a vet as soon as possible. Kittens that young go downhill very fast if not cared for properly, and they’re tricky.