In order to be able to enter the pool of candidates, you need to first qualify for one of the programs.
Does a letter from my employer count as a job offer for Express Entry?
For Express Entry, a job offer is defined as a process under the PNP or LMIA.
Can I apply for Express Entry even I do not have a job offer?
Yes, although candidates with a job offer have better chances to be invited to apply for permanent residence; through out the year the scores for Express Entry change and people without a job offer may be called as well.