You should keep any new cat in for at least a couple of weeks, so that it gets used to its home surroundings before letting it out. Also, it depends on how old your kitten is. I'm assuming it's over 8 weeks. Has it had its innoculations? I'd advise you to get them done first, before letting it out, as if it gets cat 'flu or any of the other things that can be vaccinated against, it could become very ill or could even die - young animals are obviously more susceptible as their immune system is weaker than that of an adult. When you do let the wee thing out for the first time, keep an eye on it and make sure it is able to get back indoors in a hurry if it needs to - it could get frightened by something and panic. Most likely, though, when it gets its first taste of the outdoors, it'll be fascinated by all the new sights & smells (another good reason to watch over it - you'll be in stitches at its antics!). Have fun with your new baby!