The weight limit is around 35 lbs for all slings (there are other types of baby carriers that don't have this limit, but I'll stick to slings since that's what you asked about).
Here are the basic decisions you need to make :
1. Do you want a padded or unpadded sling? Padded slings are typically hard to adjust, but the padding makes it easier to nurse in a sling. If you aren't sure, I would recommend NOT getting the padding. An unpadded sling usually has an "Open tail" which means the fabric that feeds through the rings has not been sewn together. That means you can adjust the top rail and bottom rail separately. If you want baby to feel secure, this is essential.
2. Do you want something adjustable (a ring sling) or nonadjustable (a pouch)? A pouch is very portable and handy. The downside is you need to make sure you get the right size, you won't necessarily be able to share it with anyone. The downside to a ring sling is you have to re-adjust them every 1/2 hour or so (depending on the sling) because the fabric sometimes slips a bit, and they aren't as portable because they have rings.
3. Do you want to be able to nurse in it? If so you should probably get a ring sling, because nursing isn't quite as easy in a pouch (at least that's my understanding).
4. How much do you want to spend? If you can spend $100 on the "cadillac" or slings, you can get a Mamma's Milk pouch at
www.mammasmilk.com or a Zolowear ring sling at
www.zolowear.com. If you want something cheaper, there are a multitude of options, ranging from around $30 and up.
I got a ring sling from TaylorMade (
www.taylormadeslings.com) and I love it. Reverand Jan at Sleeping Baby makes a really nice sling pretty cheap at
http://www.sleepingbaby.net/.
The main thing to keep in mind about slings is that there are a ton of WAHM's that make them (the two sites I listed are both WAHM run). Consequently, if you have a special fabric you want the sling to be made of, or have a special request, or just need some help determining what works best for you, all of the ladies are really good about working with you.
Hope this helps. There are a TON of options. At
www.mothering.com there is a Babywearing forum that has reviews, and at
www.thebabywearer.com there is a Reviews section, as well as forums where you can ask questions.
I have recently gotten into this whole thing and have found out a lot, but I am far from an expert.
Hope this helps! Let us know what you decide to get.