If I signed a lease for a house which is effective today and the previous tenant’s lease has expired but he refuses to leave, what are my options?

Unfortunately, you cannot yourself evict the tenant--you are not the owner of the home. On the landlord can evict.

What you can do is tell the landlord that he is in material breach of the lease by not providing you with possession of the home on the agreed upon date. Also inform him that you have only 3 days to vacate your current home, and if the landlord does not provide you possession--i.e. get rid of the current tenant--before then, you will not only consider the lease terminated, but also sue the landlord for all your out-of-pocket expenses, including a hotel or motel; moving and storage for goods; if you end up renting a different (but comparable) home for more money, for the difference in rental price, etc. You can certainly be very low-key about the above--no need to shout and bluster--but you should put the landlord on notice that he has already violated his lease and that you will hold him accountable for all costs flowing from such violation. That will hopefully motivate the landlord to take action--though even if he does, he will not likely be able to get anything done in this timeframe--and also possibly motivate the landlord to help you find (and he should pay for) some short term residence while he evicts the tenant.

If there is no satisfactory resolution,  then you will need to find some new place to stay on your own, then sue the landlord for your costs.

When you contact the landlord, don't just speak with him--also send hm something in writing (e.g. an email, a fax, a fed ex, etc.) some way you can prove delivery, so you can later show a court, if need be, that you made the landlord aware of the situation and the constraints (about to lose your current residence) on you.