Addition is one of the basic math skill. Work on different addition skills like addition of two numbers of one digit, of three numbers of 1 digit, with zero, of 2 digit number with a 1 digit number with/without carry, of 2 digit number with a 1 digit number with/without carry, of two numbers of 2 digit with/without carry, of three numbers of 2 digit with/without carry and much more.
Since practice makes perfect, one effective method is to use your text book and write on paper all of the concepts along with equations that represent that concept. Keep each concept on a different sheet. Take notes and write down your opinion for every equation or algebra concept. You may not like it at first, but the more you practice and use this personal algebra notebook the better for you. You must be patient because at first it might seem like you are never going to be able to figure out how all of these numbers plug into one and other, so do not rush, remember that learning algebra takes time.