If you've never used it before, or only used over the counter retinols then I would start once a week for a few months and then slowly increase to twice a week etc. Start slowly or your whole face will likely peel off.
You only need a very small pea sized amount, and try to avoid places like around your eyes where the skin is thinner. I also avoided using it on my neck for a long time because it made my skin there way too irritated - I looked like I had hives. I can tolerate it now, but that's only after a few years.
Honestly, my best advice would be to be patient. I know it's difficult because you want it to work right now, but tretinoin is a long term game. It will make your skin better, but only if you use it properly over a longer period of time. Don't expect good results for a few months.
Also be prepared for irritation and redness, although everyone is different and some people don't get it that badly. I got it quite bad for a few weeks, with peeling around my nose and mouth. Thankfully I just wore scarves up around my face in work (this is a good time of year for that). I'm not trying to scare you off, but it can be a bit of a nightmare for a while so try to persevere.
Also, please make sure you use sunscreen everyday and don't use other harsh serums on your skin - like don't overuse glycolic acid or similar or else your skin will be too irritated.