Yes, you can begin with a sincere sankalp from tomorrow, but formal Varshitap should ideally be started with proper guidance from a Jain sadhu/sadhvi or knowledgeable guru.

In Jain tradition, Varshitap is a very serious tapas and its beginning, count, and observance should be taken carefully according to the proper vidhi and your capacity.

So the simple answer is:

• For personal spiritual intent: yes, you may start tomorrow.
• For formal Varshitap observance: start it with proper Jain guidance and correct vow-taking.

If you are not fully sure about the vidhi, it is better to begin only after understanding it properly, so the tapas remains pure and sincere.