"It's a little rainy" is better sounding. This would mean that, during the course of a day, it rains for a few minutes to a half-hour...then the rain stops, but it's probably still cloudy...then it rains for a few minutes again...then there's a couple hours with no rain...and so on. For this type of weather, you could also say, "it's been raining off and on today." If you mean it's rained, but there wasn't much precipitation, you could say, "we had some light rain" or "it rained a little." If you've had several dry days during the past several weeks and you needed rain (for the farms or some other reason), and now, you've had the rain you needed, you could say, "we've had some rain." All of these examples are nuanced. I come from an area of the world (Midwestern United States) where we really like to talk about weather. Weather is important and interesting to us. Therefore, we use a lot of words and phrases here that other English speakers may (or may not) use. I hope my comments are helpful.

You can say "rainy a little". It's either rainy or it's not. To me, "rainy" itself is somewhat vague. What are you trying to say? a. Did it rain already and everything is (soaking) wet outside? b. Has it NOT rained yet but it looks like it might rain? I would say: - It's drizzling outside. (misty raindrops falling; very slight rain) - It's raining outside. (everything's wet) - It's raining cats and dogs outside! - What nasty weather outside! (rainy and windy with lightning and thumder) - It looks like it may rain soon. You'd better take an umbrella with you when you go.

Didn't you ask this question several days ago?