I'm a former foster and current adoption coordinator for a rescue cat organization. I got involved for the same reason you did - I wasn't ready to adopt after the loss of my former Best Little Friend and I thought this would be a great way to make a difference and not feel so empty at home. One difference is that I started doing it right as I FIREd, so I was home all the time when I fostered. We fostered 4 kittens, the last two of which we adopted (which I was upfront with the org about - that I'd eventually want to adopt again). I hope to foster again, but have to work with my new landlord to see if he'll let me have more cats around.
The other advice above is great. What I'd specifically suggest is that you try fostering older (1 year+?) cats, especially at first. Kittens require a lot more attention and can grow terrible habits more easily when you aren't home. (Well, they can form terrible habits even if you are, see, e.g., my now 10 month old monsters!). They need a lot more interactivity and attention than older cats, and being gone for that long each day is much less desirable for young kittens. In addition, the younger you get the more you have to be with them every few hours. We started fostering our second set (our current adoptees) at 3 weeks and they needed to be fed and burped (<---yes, burped! It's totally as cute as it sounds to burp a 3-week old kitten) every 3-5 hours. Young kittens need this up until....6-ish+ weeks before they are fully transitioned to solid food and only two meals a day (doable with being gone for 12 hours). They'll also be closer to litterbox trained by then, which will mean you won't find messes that have been sitting, soaking into your furniture, floor, etc. for 10 hours before you are able to clean it up. Oh, and the impromptu bathing required because they are covered in poop or who knows what doesn't always coordinate easily with a busy work schedule!
But an older cat would be great! They do need socialization and can be tougher to get there than kittens, so your help will be much needed there. We have foster families in my group that love taking older cats and some that only take bottle babies, so I would think most groups would work with you to find age appropriate cats for your circumstances.
Also, I'm sure you've thought of this, but get to know your foster org's policies on what they pay for and don't. Most pay for any medical care needed at all, but if something that comes up is not an emergency, may need you to get approval before going to a particular vet they have a relationship with (i.e. maybe not the vet you use for your current cat). Some will give you food supplies and some will not. Some have you drop your foster off at adoption events and you may never see them again if they get adopted. Mine doesn't adopt on-site (they have an adoption/interview process) and the foster family actually gets a veto in the adoption application. Those are the sort of things that can vary that you'll want to understand.
I absolutely love fostering and am really glad to work with orgs that get ferals off the streets when possible (to help them and the local bird population) and to always fix everybody they can to try to control the population out there, which sometimes feels like a Sisyphean task, but still needs doing. I hope you enjoy!