Lunch and dinner are verbs in Portuguese
In English you have lunch and you have dinner: a general verb does the work and the meal is a noun. Portuguese usually skips that step. It has verbs that mean the whole idea on their own: almoçar means to have lunch and jantar means to have dinner. You don't need ter or any other helper verb.
English speakers tend to translate word for word and produce sentences like tenho almoço. Portuguese speakers will understand you, but it sounds odd. Tenho almoço suggests that you own a lunch, for example one packed in your bag. Once you get used to treating lunch and dinner as actions, your Portuguese will sound much more natural.
Replace the English pattern have + meal with a single verb and conjugate it like any regular -ar verb: eu almoço, nós jantamos, eles almoçaram. Time, place and company attach directly to the verb, just as they would to any other action.
A useful rule of thumb: if you would say have lunch or have dinner in English, reach for the verb in Portuguese.
almoçar covers everything English expresses with have lunch, eat lunch or grab lunch.
The verb can also take a direct object, which is the food you ate. English needs a longer phrase for this: have something for lunch.
jantar works the same way for the evening meal.
The afternoon snack follows the same logic. The verb lanchar means to have a snack, usually in the late afternoon.
There is no common single verb for it. In Brazil people say tomar café da manhã or simply tomar café. In Portugal people say tomar o pequeno-almoço. Here the verb is tomar, not ter.
O jantar means the dinner itself, so the same word can be the meal or the action. The article tells you which one you are looking at.
In the past tense, eu form, the ç of almoçar becomes c before e, so you write almocei. The pronunciation stays the same.
When English says have lunch or have dinner, Portuguese uses one verb: almoçar and jantar, and lanchar for a snack. Never build these with ter. Breakfast is the one meal that still needs a helper verb, and that verb is tomar: tomar café da manhã in Brazil and tomar o pequeno-almoço in Portugal.