Start by watching Nadine Baggott's videos. She has a few on retinoids.
If you're just starting off, then you need a mild entry grade (granactive retinol, or something lying at around 0.3% of retinol would be fine)
Lots of MNers use prescription services from Dermatica or Skin and Me. They tend to immediately start off on Tretinoin (pure vitamin A) which is only available (technically) on prescription and will be at a % initially of around 0.025.
Medik8 (crystal retinal) is a step up from retinol and highly thought of, but again start low.
It basically goes:
Granactive
Retinol
Retinal/Retinaldehyde
Tretinoin/Vitamin A
In order of strength
All of them are very much a long game thing. You are not going to look in the mirror after 10 days and see a difference. A couple of months in, you will. None of them do what Botox does, what they do is help give you the best skin for your age. You're not suddenly going to look 20 if you're 40. You're going to look 40 with amazing skin.
I used these:
The Ordinary Granactive 0.5% (granactive packaging can be deceiving- it may say 1% etc but GR needs to be divided by about 10 to get the % of active ingredient)
Inkey List 1%
I now use Tret 0.05 but will go back "down" to a retinal when this finishes I think.