From the menu, go onto your stats page and take a look on the right hand side. You will see your current challenges listed here. You will have three active at any one time.
The goal is to complete 25 of these in total. Some of them you may simply unlock organically as they appear such as get 50 kills with a blaster, however other ones can be a little more tricky and require planning.
If you come across a challenge you don't like the look of or feel would take too long to complete, you can hit
to skip this challenge which will cost you 500 credits. (please note, skipping a challenge will not complete it, unlike challenges on Plants VS Zombies: Garden Warfare for example, it will simply give you a different one)
I left this trophy towards the back end of my gameplay after i'd had the time to experience the game and modes that it offers. Just go through all of the easy ones and skip every time you come across a tedious or time consuming one.
It's probably worth noting that if you're part way through a challenge it's probably worth seeing it through until the end rather than skipping unless you REALLY don't want to complete it.
If you need any help, have any questions or if I have left anything unsaid please comment and good luck!