If you’re look at becoming a courier, it’s courier work you need to get started. So you’ll need to look at all possible sources of work, courier companies, local companies using couriers, courier work exchange sites, and nowadays there are also websites that allow “end users”, ie the actual customer, to list work for couriers to quote for.
There are others, but a good example is DeliverySupermarket.com, which is a sister site to mtvan.com. The difference between them is that on mtvan.com the work is generally trade only; ie for couriers to sell work to couriers.
(Just to be completely transparent about it, as is made very clear elsewhere on here, we are connected to the websites listed above).