It's not difficult, but you have to be disciplined. I worked three jobs in college, was on the track team at a DII college, an RA, and took 18 credits per semester and it actually worked in my favor.
You may find that, unlike many of your peers in college, you do not have the illusion of excessive amounts of free time where you can just drink and play games or take naps or scroll through Facebook. Instead, you'll have to develop a schedule for yourself, blocking out time for pretty much everything during your day: eating breakfast, travelling to work, studying, etc. and then of course, time for you to have fun as well.
The good news is that, while you won't have as much free time as most others, you will be setting yourself up for life after college and will likely be able to keep yourself on track better than someone whose only responsibility is school. If you do it right, your time management skills will be top-notch and they'll easily transfer into post-graduate life, making you more valuable to employers.
If you can swing it, on-campus jobs are a really good option. I worked as a tutor, a Resident Assistant, and was an official and supervisor for the Intramurals program at my university. Some states/universities limit you to 30 hours a week for an on-campus job, but usually that's enough to help get you by during the semester.
Please let me know if you have any more questions!