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.
Others things to look for:
Look for rack mounting points (small threaded holes on both sides of the bike) near the seat and near the rear axle if you plan to carry cargo. e-bikes are great for daily shopping trips and it is highly recommended you consider a rear rack and panniers (bags) for it. Top rack mount points can be replaced with a replacement seat clamp but bottom clamp points require P clamps or similar that may not be as strong as built in mount points.
Separate brake and shifter controls. Combined brake/shifter levers can often make it difficult to mount additional controls to your bike that are needed when converting it to an e-bike, mirrors and turn signal controls may not fit well with combined brake/shifters and make it more expensive to change as you need to buy replacements for both.
Kick stand mount plate near the crank to mount a double kickstand. It may be difficult to mount a kickstand without a sturdy kickstand mounting plate on your bike. E-bikes are often too heavy for single kick stands mounted near the rear wheel.