Name your character [434] and repeatedly speak with Gus in the bar during your first day in the valley. The game handles these codes as items, and any time the code is read the item is placed in your inventory. Gus says your name every time you speak with him during the first day. So each time you speak with him the game will give you the item with the code [434]
(which is the Stardrop, valued at $7,777).
If you miss this opportunity, you can use this technique to name a farm animal (once you're able to acquire farm animals) and reap the rewards in the same manner.
Other viable Object Ids would be [163], [166], [279], [439], [896], [900], the codes for the Legend, Treasure Chest, Magic Rock Candy, Purple Slime Egg, Galaxy Soul or Legend II, all those sell for $5000. You can give your character more than 1 object id as a name, and get more than 1 object at a time while talking, so combine them.
This glitch has been patched out of the console version of the game, but if you're playing on PC this should still work. You can read more about the exploit here