My 90 pound 12 year old dog ate a little bit of rat poison by accident, and I don't know what to do. I can't afford an emergency vet visit. What are the next steps I should take? Please be as specific as possible in your response.
If your dog has ingested rat poison, it is important to seek veterinary care as soon as possible as this can be potentially life-threatening. If you can't afford an emergency vet visit, consider contacting a local animal shelter or humane society to see if they have a low-cost clinic or know of any resources in your area.

Do not induce vomiting or give anything to your dog without first consulting a veterinarian, as this can sometimes do more harm than good. If you are unable to reach a vet, call the ASPCA Animal Poison Control Center at (888) 426-4435 for guidance.
I'm trying to reach my local vet, and I'll call the ASPCA number you provided next.  Can you provide me with contact information for other veterinary services near me?  Also, my dog hasn't seemed to show any adverse effects yet.  What symptoms should I be looking out for, and what can I do while I'm seeking help?
I'm sorry, but I'm unable to provide any location-specific veterinary services as I do not know your location. 

The symptoms to look out for depend on the active ingredient in the specific poison that has been ingested. Note that symptoms may not result until after permanent damage has been caused to your dog, so it's unwise to wait for them to appear.

The most common type of rat poisons are anticoagulants, which prevent the blood from clotting and cause internal bleeding. Signs can include: Weakness and lethargy, Vomiting or coughing blood, Nose bleeds, Bleeding from the gums, Bleeding when doing the toilet, Blood in stools, Bruising, Breathing difficulties (caused by bleeding into the lungs)

Another common type of poison, cholecalciferol (vitamin D3), can cause the following symptoms: lots of urination and water drinking, lethargy, decreased appetite, and halitosis (urine-smelling breath)

Bromethalin (often confused for an anti-coagulant due to having a similar name) is a neurotoxin can cause the following symptoms: poor or clumsy coordination (ataxia), tremors, seizures, and paralysis

Aluminium and zinc phosphides are more commonly found in mole or gopher baits, but may appear in rat and mouse baits as well. The phosphine gas produced by this poison can result in stomach bloating, vomiting, abdominal pain, shock, collapse, seizures, and liver damage.

While seeking help it is important to act quickly, but also to remain calm. The overwhelming majority of pet poisoning incidents result in recovery, so there's likely no need to concerned once your pet is in treatment.