My big toe is paining since a week & when i look closer using a bright light i can see yellow pus kind of formation beneath or beside the nail,help!

Have it drained: This is likely what is called a paronychia, and it's an infection along the side of the nail. You may have trimmed a nail too closely. (Or could have sustained a foreign body). You can soak it in warm, soapy water. But if it doesn't improve soon, a physician will need to open it to drain the pus. Antibiotics are not especially helpful with pus collections like that. Drainage is the key.

Ingrown toenail: You are describing what sounds like an ingrown toenail or paronychium. You should be senn by a physician or podiatrist. In the mean time warm soaks in epsom salt bath and then gentle pulling of the skin away from the side of the nail may relieve the pain and allow the infection to drain.