You can try ab initio prediction at the Robetta online server. However, if your protein is large (>120 residues) you cannot expect to get a valid result. In case of homology modelling, you need to check whether you have templates. A sequence search against the Protein Databank should help you with this.
As Devon said though, this is far more complex than you might think. Identifying valid templates is not always straight-forward. In case you're attempting to use ab initio modelling, it makes it even harder because it's difficult to truly believe the result you might get. I would suggest you do a little reading around the topic before you start!