Assume it will be extremely difficult.
To change from a student permit to a work permit, you will first have to find an employer who is willing to give you a full time job. That employer will then have to advertise the job to prove that no Canadian could be found for the job. (If a Canadian can be found, then the employer must hire the Canadian - not you.) Once the employer has advertised the job to prove no Canadian can be found, the employer must then apply for a Labour Market Opinion (LMO). If the LMO is approved (not all are), you can then apply for a work permit.
It costs money and takes several months for an LMO to be processed. For this reason (and because there's no guarantee the LMO will be approved) many employers are not interested in hiring foreign workers.
To be approved for a work permit, you will have to prove that you have strong ties to your home country and have no plans on remaining in Canada long term. If you came here on a student visa and are then changing to a work permit, CIC may believe that you are just looking for a way of staying in Canada and it's possible it may be difficult to get a work permit approved.
