Lean beef, chicken, fish, and eggs are some of your options. Tofu, nuts, and seeds have protein, too. Try to get about 5 to 6 ounces per day, although the ideal amount for you depends on your age, sex, and how active you are. When you don't eat enough of these foods, your body makes more of a substance that binds with testosterone, leaving you with less T available to do its job.
The diagnosis of late-onset hypogonadism requires the combination of low serum testosterone levels with symptoms of hypogonadism. Questionnaires are available which check for the symptoms of hypogonadism. These have been validated for the assessment of aging patients with hypogonadism (Morley et al 2000; Moore et al 2004) but have a low specificity. In view of the overlap in symptoms between hypogonadism, aging and other medical conditions it is wise to use a formal method of symptom assessment which can be used to monitor the effects of testosterone replacement.
To get your levels into the healthy range, sun exposure is the BEST way to optimize your vitamin D levels; exposing a large amount of your skin until it turns the lightest shade of pink, as near to solar noon as possible, is typically necessary to achieve adequate vitamin D production. If sun exposure is not an option, a safe tanning bed (with electronic ballasts rather than magnetic ballasts, to avoid unnecessary exposure to EMF fields) can be used.
Nearly 1 out of every 4 men over age 50 experience the pain of losing the ability to perform sexually as a result of erectile dysfunction (ED). Common causes of ED are atherosclerosis, diabetes, prescription drug use (namely high blood pressure, depression, and allergy drugs), and—you guessed it—low testosterone. Supplements that may help include the following:

Fenugreek is often found in Indian, Turkish, and Persian cuisine. Multiple studies have found it to improve testosterone levels, and in particular, sexual performance. Scientists at Babu Banarasi Das University and King George’s Medical University in India have found that fenugreek improved testosterone levels. Testosterone levels increased for 90% of the volunteers, sperm morphology (the size and shape of sperm) improved for 14.6%, and more than 50% of volunteers experienced improvements in mental alertness, mood, and libido.

Sharma, R., Oni, O. A., Gupta, K., Chen, G., Sharma, M., Dawn, B., … & Barua, R. S. (2015, August 6). Normalization of testosterone level is associated with reduced incidence of myocardial infarction. European Heart Journal, 36(40), 2706-2715. Retrieved from https://academic.oup.com/eurheartj/article/36/40/2706/2293361/Normalization-of-testosterone-level-is-associated