There are two main types of proper MTB Flat Pedals, Composite (plastic) and Metal.
The shop will have likely fitted some generic plastic commuter pedals on the bike as stock, these will not be up to the job as not enough grip and poor build quality.
You do not need to spend a lot of money on decent pedals, as a lot of the composite options out there are some of the best pedals on the market. For example the HT PA03A's are the some of the cheapest, but also best pedals in terms of grip and performance. Pedal choice comes down to personal preference, and a good pair of grippy shoes is as important as the pedal itself.
Spending a lot of money doesn't necessarily mean you are getting a better pedal - a lot of the very expensive ones are aimed at weight weenies who want the lightest components, but this isn't really relevant to EMTB.
If you have big feet a larger pedal is worth going for.
Here is a good article listing out some of the better pedals on the market right now - there are a lot of other good pedals though that are not mentioned in the article.
Best mountain bike pedals | 18 of our favourites tried and tested
Here is a pic of some of my pedal collection, from left to right:
HT PA03A, One Up Alloy, Unite Components Intend, Burgtec MK4, Crank Brothers Stamp 3
Apart from the One Ups which take a slightly different approach, you can see that the basic "ingredients" of the pedals are big pins and a wide platform.
I would recommend the HT's as a good place to start, and they come in lots of colours, and can be had for about 35 quid
HT