Digimon World

If you answer no to the first question, you'll get Agumon as your starter. If you answer yes, you'll get Gabumon. Agumon is a bit easier for a starting player, if it matters.

Talk to everyone in the town before you leave, and again every so often. It's usually useless, but sometimes, it sets off a flag for a quest, or gives you items.

The only place you can sleep is inside the starting house, and you have to use the menu option. Using the chair is really just a free (but slow) heal.

Press triangle while in battle to throw healing/MP items at your monster. This is crucial pretty much constantly.

Your first order of business should be upgrading the meat farm and opening the item shop. If you're lucky or experienced, you can do both on the first day, but it's alright if it takes you a couple days.

Opening the item shop: There's a coastline a couple screens from the start. (Specific directions:
From Jijimon's house, go South, West, South, Southeast, East
) In the later afternoon, there's a shadow in the water. Talk to it, then go back around the newly-opened bridge and talk to it again. It will open the item shop, where you can now get healing items.

Upgrading the meat farm: In the southern part of the starting plains (Specific directions:
From Jijimon's house, South, West, South, South
) is a 'flower'. Talk to it until it fights you. This is the first 'boss battle', so make sure your monster is healed up and you're stocked on items. Beat it, and from the next day on, the farm will give you Giant Meat instead of regular meat. This is more filling, and increases your monster's weight more efficiently.

Once those are done, you have a bit more freedom to explore, since you have better free food and a consistent source of healing items. Good places are the starting area, the cave in the west, and once you're a bit more confident/stocked up, across the east bridge.

At the end of day 3 or 4, your Rookie will evolve into a Champion. Try to be somewhere near 30 weight when that happens, or you're very likely to get Numemon, who is utterly awful.

Sometime around day 10-13(ish), your Champion will either evolve again, if it meets the prereqs for an Ultimate, or keel over if it doesn't. Don't feel bad if your first Champion dies, it happens to everyone.

Despite appearances, it's impossible to permanently screw yourself over. Worst case scenario, your monster dies and you have to raise a new one, but your town improvements will remain.