Some people may eat me alive for saying this, but when I first got started with macro, I grabbed a 70-300mm Tamron with "macro" mode. It's not insanely sharp, but it is cheap (I got mine for less than $100 used). That will take you down to 1:2 (half life-size) and let you get your feet wet. The "macro" mode goes from 180-300mm, which is perfect for critters which are skiddish or things that bite/sting. I have about 2 feet of working distance. The image quality wide-open is kinda "meh", but stopped down to f/8, you can actually get some decent pictures. It wasn't uncommon for me to shoot even as high as f/20 and still get good images (the lens goes to f/45 at the 300mm focal length).
If you are SURE that you want to do macro, then I'd say skip the 70-300mm and go with a dedicated macro lens so you can get down to 1:1, or better. But if it's something you want to try, a cheap, used Tamron "macro" zoom lens is great to get started with and it won't cost you an arm and a leg.
The lens I got wasn't even the newest model, it was one of the older ones...the ones that take like 15 sec to go from infinity to close focus, lol (okay, maybe not THAT bad, but it's pretty bad). I really liked having a lens that I could use for many things, "macro", telephoto, portraits, walking-around, etc., on my camera at the same time. I still use this lens a LOT. Mostly because I can't afford a bunch of different lenses right now.
Almost all of my "macro" shots are taken with this lens:
https://secure.flickr.com/photos/60637962@N03/
If you have a 3xxx to 5xxx series, you won't have autofocus (which really isn't a problem w/macro, since you tend to manually focus anyway), but the cameras with a built-in focusing motor will focus just fine with the older version. I don't know if the newer version of the lens has the AF motor built-in or not, but I used it on my D3100 and D5200 just fine before I got my D7100.
Personally, I wouldn't buy anything less than a 90mm for critters, you will just be too close to them and you'll find that you scare 90% of them away. That's one thing that I truly LOVE about the 300mm focal length...I have a lot of working distance. The 70-300mm is NOT a true macro lens, which is why I've been putting the word macro in quotes. To be a proper macro lens, you need 1:1 or better reproduction, the tamron only goes down to 1:2.
Anyway, good luck and I look forward to hearing about the lens that you decide to get.
