Typical ruleset to open a few ports and block the rest.

iptables -P INPUT DROP
iptables -A INPUT -m state --state ESTABLISHED
 	-j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -p tcp --dport 25 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j ACCEPT