The usual recommendation, until you become familiar both with the concepts and the ways they can be implemented in RouterOS, Is to use the default firewall that is used on SoHo devices, see Rule #8:
as a base, then add or modify filter rules to suit your specific needs.
Starting from scratch is dangerous.
Firewall filter rules are applied from top to bottom within a SAME chain, so to keep then more readable they are usually written grouped by chain.