you want to make sure that you have adequate amounts of protein in your diet. Plant protein sources are plenty, and here is a link to more information about that http://www.happycow.net/vegetarian_protein.html. Keep in mind that, as an active person, 1.5 grams of protein per kilogram of body weight are sufficient. To improve utilization of protein, try to have about 30 grams per meal.
Are you vegetarian or vegan? Vegetarians tend to eat eggs and dairy products that are loaded with protein. I would need to know exactly what you exclude from your diet to properly answer the question.
In order to build muscle, you need protein. This is the basic building-block of muscle.
You also need to make sure you are eating enough of the right kinds of carbohydrates to keep up with your energy demands. Carbohydrates are the preferred energy source of the body. If you aren’t eating enough “Slow-burning” complex carbohydrates (starches), your body will convert protein into carbohydrates, which is undesirable. Good sources of complex carbohydrates are whole grain bread, brown rice, potatoes, sweet potatoes, squash, and any other whole grain.
Back to protein:
Eggs and dairy products are protein rich. Qinoa (a grain), amaranth (another grain) and hemp seed are all complete protein sources. Other plant foods have some of the amino acids that can be assembled to form complete proteins in your body but need to be complemented by other plant foods that have the missing amino acids, hence the term, “Complementary proteins”.
Since this is your lifestyle, you should probably know this subject inside and out. The more you know, the easier it will be to make healthy lifestyle choices. That said, a good rule of thumb is that “grain + legume = complete protein”.