You'll learn in Castle Coneria that GARLAND can be found to the northwest. Indeed, that's the only direction you can travel at this stage of the game. Once you're fully equipped, it's time to begin your journey.
While wandering the countryside, you are liable to be attacked by a group of IMPS. These creatures offer very little resistance but will serve to demonstrate an important battle mechanic. Unlike in later games (and indeed, most RPGs made in the past 20 years), your characters do not change targets if the monster they were told to attack is defeated. You'll want to learn to spread out your attacks, and get a good feel for how many attacks are likely to be needed to take a given creature down. This is arguably the most important tactical consideration for Final Fantasy combat.
There are two schools of thought as to how to proceed at this point in the game. The conservative approach is to fight battles on the World Map North of Coneria until you reach level 2 or 3, saving up money for more spells, perhaps some HEAL Potions or a PURE Potion, and money to save at the inn. This is effective, but slow-going. The more aggressive path, which I recommend, is to head directly to the Temple of Fiends, walk right up to GARLAND, and challenge him at level 1. You may lose a warrior or two, but GARLAND drops 250 G and will bring any (surviving) party member to level 2. Once he is defeated, you can talk to Princess Sara behind him, and she will teleport you directly back to Castle Coneria. You will have to make a return trip to the Temple of Fiends if you want to collect the treasures there, but this will get the game started off much more quickly.
GARLAND
GARLAND is the first boss in the game, and easily the most dangerous foe you've faced so far. He hits hard, with the potential to take out level 1 mages with a single attack. He also has 106 HP, so the battle will last several rounds. At this point in the game, you don't have a lot of options: just pound away with melee attacks and whatever attack spells you have available. This fight can be won at level 1. GARLAND may take out a party member or two, but he drops enough G to bring them back at the Clinic in Coneria with plenty to spare to heal and save at the Inn.
Talk to Princess Sara after defeating GARLAND and she will teleport you directly back to Castle Coneria. Don't forget to speak to her to obtain the LUTE, and speak to the King so he will create a bridge leading north from Coneria. Many of the residents of the castle and town will have new things to tell you now that the Princess has been rescued. In particular, take note of mentions of Matoya to the north and Pravoka to the east.
Temple of Fiends
There is more to find in the Temple of Fiends than just GARLAND. There are four treasure rooms, one in each corner of the Temple. The eastern rooms are locked with the mystic KEY, but the western rooms are open. You will encounter a lot of Undead in the Temple, so you may want to buy HARM for your Wh.MAGE. If you can spare the G, bring a PURE Potion or two as well, just in case.
Many encounters in the Temple of Fiends are straightforward. BONES, SPIDERS, and ZOMBIES are basic foes with no tricks. There is a common encounter with a lone GHOUL, a dangerous Undead foe that attacks three times and causes Paralysis. Your HARM spell slots will be put to good use against them, since your FIGHTER is liable to end up Paralyzed and unable to fight back. Save FIRE for CREEPS. They appear more rarely, but you may see two at a time, and they have a powerful attack. GrWOLVES are also an offensive threat, but they lack the defensive capabilities of CREEPS. The most dangerous foes in the Temple of Fiends are WrWOLVES, though they are mercifully quite rare (appearing in 1.3% of all encounters). They are statistically similar to GrWOLVES, but their attacks cause Poison, which is a death sentence if you don't have a PURE Potion.
You'll find a Cap in the southwestern treasure room. These offer a minor boost to Defense, but they are the only head gear available at this point in the game so they are a good option. Your FIGHTER (and RedMAGE) should be well-protected already, so you may want to give the Cap to either your second-slot character or your healer.
The northwestern treasure room has two chests, containing a CABIN and a HEAL Potion. The CABIN is a nice find, but probably a bit excessive for this point in the game. If you bought any TENTS, they are more cost-effective (especially if all you want to do is save). If you decided to grab the treasures before fighting GARLAND, you might want to use the CABIN to gain any hit points gained by increased levels, then immediately return to the Temple for the boss fight.
You can't open the other two treasure rooms at this point in the game, so your work is done after collecting the first three treasures and defeating GARLAND. You only get one free teleport back to Castle Coneria, so you'll have to walk back if this is your second trip.