When start playing FarmVille you will have a little farm, plants a most profitable seed by calculating it to an hour, the seed which can be harvested in 4 hours are good to plants. try to upgrade your farm using a coins, keep farm cash to use for upgrade a farm at bigger size.
There is a way to get XP faster, If you have enough coins you can buy hay bale it cost only 125 coins. Another way to gain XP is to plow a land and delete it, this will cost only 15 coins you can repeat plowing and delete it as much as you want
you can get 100 XP using only 1500 coins.
A tree are also good to invest, it use very small land to plant. of course there are no XP at harvesting, but you can earn a lot of coins from good profitable tree and use that coin to gain XP.