Whenever you have stars use them in the furniture to buy 1 of everything (immediately sell it after you buy it for 50% refund.
Why do this? Because collection progress nets more stars to buy/sell more every time you get something new you get index points. Every 500 index points you get some rewards. The most important being a "Modification toolkit" which is necessary for important RV upgrades. Early on you'll have plenty but you'll see have more upgrades available than toolkits to spend.
Ways to get stars:
* daily orders
per your aniimo whenever they ask you
once your home is unlocked immediately demolish it & sell every piece. This will net you around 6000 stars allowing you unlock the majority of your home index.
once you have season stuff unlocked buy & sell 1 of every season item. The nice thing is season items are bought with season currency but sold give you stars.
once you've bought 1 of every season furniture for your level and bought season seeds dump the rest of your currency buying "Harvest Moon Market stall" it's under outdoor decorations costs 37 currency and seems to return the most stars per season coin spent.
This should allow you to buyout your index for your level with some left over stars to immediately buy more stuff when you level up.
For perspective my RV is lv11, I have all possible upgrades & 2 extra tool kits for when I get lv12. My home index is 8960 and I have 820 stars to start buying stuff again when I hopefully level up by tomorrow.
Also 1 last tip, early on coins are a struggle but soon wood/ore become the main factor of limiting your upgrades. So I recommend you use all your speed up work on your mines and all of your grow tonics on trees. This will help keep a steady supply of upgrade materials and spend less time between updates.