What's the difference between 我去吃飯 (去 + Action) and 我要吃飯了 (要+Action+了)?
我吃飯 =  I eat dinner (meal)
我(去)吃飯 = I (go) eat dinner
我(要)吃飯 = I (have to/ want to) eat dinner
我(要)(去)吃飯  = I (have to/ want to) (go) eat dinner


You can add final particle '了' (indicate change of situation) to all the above examples :


我吃飯了 =  I eat dinner (meal) now- 了 indicates situation change from not eating to eating
我(去)吃飯了 = I (go) eat dinner now- 了 indicates situation change from not going to going 
我(要)吃飯了 = I (have to/ want to) eat dinner now - 了 indicates situation change from not have eat to have to eat 
我(要)(去)吃飯了  = I (have to/ want to)(go) eat my dinner now - 了 indicates situation change from not have to go to have to go 



  去 + Action vs 要+Action+了


More examples:


我看電影 = I watch movies
我(要)看電影 = I (have to/ want to) watch movies
我(去)看電影 = I (go) watch movies
我(要)(去)看電影了 = I (have to/ want to) (go) watch movies now- 了 indicates situation change from not going to going