You're approximately my build. I've always been this size even when I ate meat.
You have several options which include protein shakes (soy, rice, or wheat). Normally, to get a rough idea of how much protein you should consume each day, multiply your body weight (in pounds) by the following:
if you're sedentary, weight x 0.36
if you work out heavily, weight x 0.7
For example, let's say you really wish to bulk up and work out heavily. Do 145 times 0.7 which equals 101.5 meaning you would need to consume approximately 102 grams of protein per day.
Now, let me add 3 more items:
1) Don't consume more than 25 grams of protein in one sitting -- especially a fast absorption protein like a powder. Your body can't handle it, and you could be setting up illness in the future.
2) A lot of thin guys have a higher percentage of slow-twitch muscle as opposed to fast twitch. Fast twitch is up to 9 times bulkier than slow twitch when looking at the comparative work load. All of us are a mixture of the two, but bulky guys usually have more fast-twitch by percentage.
3) Different proteins have different body absorptivity rates. Rice is lower than soy, for example. This means you would need to eat more rice protein to accomplish the same amount of protein absorption.
However, as a word of caution to a 19 year old... soy is very high (when compared to other legumes and vegetables) in phytoestrogens. Because of the promiscuity of physiological estrogen and testosterone receptor sites, some males will notice a significant drop in sex drive as they increase consumption of soy. This will go away in time (like within 2 months) when soy consummption is stopped.