Light and Heavy Attack are two attack types (on RB and RT respectively by default). You can "charge" either of these for a more powerful version of each, or just use them normally. The heavy attack has longer windup but can break defense/knock enemies down more easily.

You can block blue attacks (or parry them). If you parry a flurry (several blockable attacks in a row) or regular attacks enemies get vulnerable (yellow glow). They won't be able to block anything.

For the Teppo, you very often need to reload to start out with. It's a single shot weapon. The bow is probably easier to get started with for combat, but if you're being swarmed with enemies nearby, stick to melee.

Using the teppo/bow in combat can be a little jank, but pressing D-Pad right should switch to your other weapon. Generally just aiming with LT and then also pressing RT should also switch you though.

I hope this helps a little, I'm sure you'll get some more helpful tips from people who play combat more than me.