It can take a few days for an animal to get used to you, your home and schedule. They may not be themselves; they may be anxious, chew stuff, bark at noises, while cats may hide or hiss. This can be disappointing but it is normal and usually short lived.
Give your foster plenty of physical space as they start to trust you. It's tempting to want to cozy up, but it can backfire and scare them. Think of it like dating. Too much too soon is uncomfortable.
Your foster may not be housebroken. Even most housebroken pets have accidents in the beginning during stressful transition periods and before they've gotten the swing of things.
Setting up a room or even just a little corner of a room just for the foster, will make them feel safe knowing they have somewhere to go if they feel overwhelmed.
You might want to show all your friends and family the little one you just brought home. However, having so many people around them could actually scare them. So introduce your family and others slowly.
Set a time where you try and feed them around every day. This will set a trust level knowing they will get something at that time. Make sure to play and spend time with them everyday so they feel loved in your home.
If they do a trick or are just listening really well, give them a treat to show them how amazing they are!