You're not giving us a lot to work with
If you're trying to understand how GLP1s work then a quick google of GLP1s Beginners Guide will get you the basics.
As you're from the UK I going to make the assumption you've heard about Wegovy and Mounjaro and might already know there's pens you can buy from online pharmacists there. Aside from being a place for general chitchat conversation on GLP1s, on this forum there are a lot of 'vendors' from China who are basically selling more or less the same drugs (Semaglutide and Tirzepatide respectively) as the pharmacies in the UK. The difference being of course that they are much much cheaper.
In my opinion (and I'm sure many agree) is that a newbie should start on Tirzepatide (Mounjaro) because it tends to have the least side effects, is very effective and is currently being used by millions of people globally. A starter dose is 2.5mg per week, but please Google dosages yourself (of any drug).
If this is your first time taking any GLP1 drug I'd highly recommend you use a legit local source in the UK first. You can buy a months supply of the starter dose legit for like £120 - there's plenty of info on this sub reddit on best / cheapest legit sources in the UK.
https://www.reddit.com/r/mounjarouk/
For the grey market... you can find the vendor section here
https://glp1forum.com/forums/vendors/ - you can search for trusted vendors around the forum, spend some time doing this as some are more newbie friendly than others (take Paypal, better customer service, but charge more).
It's worth noting that the Chinese vendors will quote something like 'Tirzepatide 30mg' this means that they will send you 10 (yes 10) 30mg doses of the drug in
powder form in vials.
One 30mg vial of Tirzepatide is equivalent to one 7.5mg Mounjaro Pen from a UK chemist, because the Mounjaro pen contains 4 doses (and 4 x 7.5mg = 30mg)
Basically a seller advertising 'Tirzepatide 30mg' is giving you the equivalent of 10 x 7.5mg Mounjaro pens.
Because the drug arrives in powdered format you'll need to 'reconstitute' it into a liquid by adding bacteriostatic water into the vial. You'll also need needle syringes to do this and to inject yourself. You'll need to buy this bac water and needles separately, I got my supplies from Amazon... but there might be cheaper places (Insulin needles are very common). Again search on this forum for 'reconstitution' or Google reconstituting peptides, it's not a difficult process.
Postage from China can take 2 weeks to a month (hence maybe you want to test the water first with a legit UK Pharmacy). The vendors post frequent testing results to prove their drugs are pure and of course individuals on this forum frequently test the drugs they receive and post their own personal results. I think it's fair to say most people on here find the whole process safe and relatively risk free, but everyone reacts differently to different drugs and of course there is always a little bit of risk when buying from a completely unknown entity in a different country.
Hopefully this is enough to go on to start - if not ask more specific questions