Should I make customers create an account before checking out?

This is an entirely personal choice but in essence there are 3 options here.

The normal (default) checkout process on our websites is

Our studies have shown this to be the optimal checkout process.

An alternative checkout process is

in our view this additional checkout step 'Account Login' (where the customer is asked to login to an account or register) can increase the chance that the customer drops out of the checkout process (and so reduce your conversion rate)

There are however certain circumstances where you want to remind the customer to login to an account, such as if you operate a reward points scheme.

On the account login page you can also choose whether customers must be logged into an account in order to complete the checkout, or whether you will allow 'guest checkout' (which is where the customer can check out without logging into an account).

Both of these settings:

i) Whether to direct the customer to the account login page
ii) Whether to allow guest checkout

can be changed in your checkout's settings. Please read this article for more information about customising your checkout.