How to know what to build

Okay so my problem is that when I go up against a mid player and I get ganked by a physical jungler I don't know what I should build breastplate and void stone or just one of the items.

Okay, so I'm guessing you're choosing a mage when you are in the mid lane. On that asumption, Breastplate of Valor is a fantastic item giving you CD and great physical protections. If an enemy physical jungler is really fed, you can go with Hide of the Nemean Lion for even more protections. It reflects a percentage of the damage back upon the damage dealer. Enfeebling Curse, the active can help you out during ganks.

I'm writing a guide for the mid lane as we speak so check it out when it's done! Also I've made a Ra guide, who practically lives in solo and mid lane. Check it out :P

Buy wards. Every time you go back to base buy 2 wards and place them near the mid camps on each side. Vision is power.

Thank you very much.

I think it is your playstyle more than your build. You shouldn't have that much protections on a mid mage, and if you aren't over extending you should be able to burst down any jungler in the game, no matter what god you are. Try playing with a safer playstyle and see if that helps.

I agree with the responses here. With some wards you should hopefully see junglers coming so they don't get the drop on you. As far as playstyle, try to stay behind your creeps and any support you may have in your lane. Most junglers can't penetrate intervening units so if you've got a wall of meat in front of you they'll likely leave and look for an easier target. If they do attack you've got some intervening units which YOU can bypass and they can't. You can quickly kill them or at least drive them off. Never chase a jungler into the jungle either. Likely they are prepared for you to follow and you WILL die. Just let them retreat. Some of them, like Thanatos or Neith with their ultimate can bypass all your defenses. All you can really do against them is try to avoid getting in their sights. Try to heal before you get to 50% or less.

Well sometimes just do it but not too far otherwise you have the backup of solo or duo lane (depends which side of the jungle) and every ultimate can be countered (you know when you're marked by Neith and if your almost death you have a little knowing that your in Thanatos ult so you can with right timing aegis it or escape abilty)

Well the never chase was just kind of a general suggestion for new players. Whether or not it's a good idea to follow into the jungle will depend on what god you are and what they are, how far away you are and if you can catch them, how much health you both have. Whether or not a ward is in place and you KNOW whether or not there are other enemies in there, etc. For sake of safety, don't chase junglers into the jungle unless you are pretty sure you can down them fast and you have enough hp to take a special move or two if they turn on you like Loki with Vanish or leave a little trap like Arachne with Broodlings. It's very situational and takes some experience. I feel it's just safer to suggest not chasing junglers at all until you get a good grasp on what you can get away with.

when you're going to chase the most important thing (why calling missing is important) is map awareness know where the other gods are.

And the sentence about catching and health is important too you will want to have him dead really quick. the best is to have him dead before he even gets deep in the jungle (never follow him into their jungle if you know others can be there) And that's why warding is so important (because it's fun too see junglers coming and instead of they attack first you get the first hits chasing them away)