This will be different in every home, but there are a few simple things to think about to start. Before you put any money at all into your house, clean it first. Purge extra stuff; put it in storage, sell it, give it away. Make room. Then have your realtor come over and give their advice. It will depend on the house and the market.
A general rule of thumb is that kitchens and bathrooms, then roofs, make the most difference to value. Your house should be sound, but it’s safe to say the prettier you can make your house, the more attractive it will be to a variety of people. Truthfully, a clean and pretty bathroom can be more important to the value of a home than a new roof. Not that a new roof isn’t important, but that first impression is valuable.
On the other hand, I do sometimes need to point out to someone who loves the Pinterest-looking kitchen that yes, it’s gorgeous, but can you smell the mould in the basement? Look after big problems, but make your house an attractive home. Not so cute that people can’t imagine themselves in it, but make it nice!