Cactus with brown patches

Hello!
I hope someone out there can help me. I bought a cactus and the guy in the shop didn't really know much about how to take care of it so a bit of advice regarding how often to water it and how much sun it should have would be much helpful.
Over the past few weeks, I have also noticed some brown patches (see attached photos). I'm not sure if it's normal or not or what I should do about it. It would be very grateful if someone could look at the photos and had any idea about what it is.
Many thanks
Steph

 

Looks normal to me.

I think it may be a euphorbia rather than a cacti, but the growing conditions are much the same, as much direct sun as possible, dry during the winter, water freely during the summer but allow to become dry between waterings.

 

I agree with Pete, to check if it needs water, I would stick my finger into the pot, and see if the compost further down feels wet or dry. Depending on location, I would perhaps suggest once every two - three weeks for during the summer months, and around six-eight weeks in the winter, perhaps even longer.

You will get to know over time what it needs, and how often, but checking how dry it is on a regular basis is perhaps the most important thing.

Hope this Helps!