Buy a bike
We highly recommend that you get a high quality bike to convert. Don't put a thousand dollars in e-bike hardware on a $200 low quality bike. It may also be unsafe to ride lower quality bikes at the speed that e-bikes very frequently travel at.
As such we recommend you check out your local bike shops for a high quality bike. Ideally one with front suspension and disk brakes.
Rim brakes if they are the linear-pull/V brake style are ok in fair weather, but if you ever bike in the rain they will massively lose stopping power. Disk brakes are highly recommended overall. Hydraulic are nice but mechanical disk brakes are still far superior to linear-pull brakes and are much less costly to maintain then hydraulics. mechanical disk brakes also allow you to easily attach e-brake levers and cheaply change the length of shiftier housing if desired.
Rear suspension can take up valuable space inside the triangle often used to mount the battery and unless you spend a great deal of money is generally of poor quality and durability. As such rear suspension is not recommended unless you plan to do a lot of off roading and plan to pay good money for it.
Mountain bikes are generally ideal and have the widest range of available components and are the easiest to convert, but road bikes and fat bikes can be converted as well if you pay attention to what components you are buying.
Do not shop at Walmart, Canadian tire or other department stores that offer extremely cheap bikes. They are often put together by inexperienced people being paid by the bike and made of the cheapest components on the market. Parts will fail, rims will warp and overall durability is very poor.