I'm almost 18, so would learning piano be too late? Also my parents are not gonna pay for lesson and I don't have money to pay. I used to take lesson 5yrs ago, but I quit. I am slow at reading bass cleft and even playing two hands challenges me. Would learning guitar be a better choice?
If you can't pay for lessons (and don't know any qualified teachers), then you're not going to have much luck – although teaching yourself is an option, you'll develop bad habits which may be pointed out in the future, when it's too late to rectify them.
In any case, the period of time it takes to learn to sing and play piano at the same time is dependent on how fast you can pick things up, and how good you are at multitasking. Practise makes perfect, and this isn't something you'll be able to do overnight.
Also, it's never too late to take up an instrument.