In my network i have some routers which are connected only to one main router(serial leased line connection) and a switch. If i use default-routing is that enought or i must use a routing protocols (etc rip, eigrp) and etc. What's the best think i should do .
When you say that the network is one main router with a serial leased line and a switch and some remote routers which connect only to the main router, then I do not see any requirement for a dynamic routing protocol. I believe that static routing will work ok for you.
A dynamic routing protocol is especially necessary when there are multiple paths through the network and you want to choose the best path and want to dynamically change depending on events in the network (if one link fails you want to choose an alternate link). When the network is a group of remote routers that connect only to the main router then a static route on the main router and a default route on the remote router will provide connectivity, will not create additional overhead. And if a link to a remote should fail then that remote becomes unreachable. I do not see that running a routing protocol would change that.