The country which issued your visa must be your main destination, not the first destination. If you enter the Schengen zone through Spain, you might be questioned, so be prepared to prove that the Czech Republic really is your main destination (length of stay, hotel reservations, travel itinerary, etc.). You can find the applicable law in the Article 5 of Schengen Acquis:
Article 5
Member State competent for examining and
deciding on an application
1. The Member State competent for examining
and deciding on an application for a uniform visa shall be:
(b) if the visit includes more than one destination, the Member
State whose territory constitutes the main destination of the
visit(s) in terms of the length or purpose of stay;
Enjoy your travel.