I want to know the total procedure of depriciation regarding Straight Line method. From beginning of depriciation charged on an asset till it sold & purchase a new one.

First you have to determine the amount of depreciation for each year. Take the cost of the asset minus the salvage value divided by the number of years useful life. To record the depreciation the journal entry will be:
Debit Depreciation Expense for the amount of depreciation during the period
Credit Accumulated Depreciation for the same amount.

Prior to the sale you will have to get your depreciation up to date. The journal entry will be the same as above but the amount may be different because of a partial period.

To recod the sale of the asset the journal entry will be:
Debit Cash for the amount received
Debit Accounts Receivable if the sale was on account
Debit Notes Receivable if a note was signed for the sale
Debit Accumulated Depreciation for the amount od depreciation accumulated on the asset
Credit the Asset for the cost of the asset.

If there is a difference between your debits and credits you will have an additional debit or credit entry. If the difference is a debit, then you will Debit Loss on Sale of the asset (asset sold at a loss) for the amount of the difference. If the difference is a credit, then you will Credit Gain on Sale of the asset (asset sold for a gain) for the amount of the difference.