roads are found in the Shopping Bag icon, under the Road icon section. i believe you start with 30 roads. they cost $10 each, so you will need $2700 for this achievement. after completing all the main chapter 1 missions you should have over $10,000. simply place roads wherever you like until you reach 300 pieces. simple!
you should also be able to afford:
Warning: keep in mind that completing missions usually require spending money, which is hard to come by at first, so you might want to save doing this until you have some extra cash. the beginning of the chapter 2 missions required about $8000 to buy a gorilla habitat and then a few more thousand to upgrade it, so i had to wait for some profits and click on my decorations for money a bit to be able to afford to continue.