Locate a good record store, preferably one that specializes in vinyl music. Go through the store and find the kind of music you like. Often the jacket cover of the record is the first thing you notice. If you don't know what you're looking picking up the most interesting jacket covers is one way to start. The more worn the jacket cover is, likely means the more it has been used. Don't let the condition of the record jacket deter you. There are gems to be found inside some of the worn down jackets.
Slide the vinyl record out of its jacket once you have made a selection. Examine the record to see that it is good condition. Vinyl is easily damaged by heat or poor care. Make sure the record isn't warped, cracked or has large scratches on it.
Listen to the record. If you have a portable turntable, use it. Some record stores have turntables you can use. If the record store specializes in servicing DJ's they will almost certainly have turntables available. However, these turntables are likely to have seen lots of use and may affect the sound of the record. Use a good, high-quality pair of headphones when you listen to the record. Listen for clear sound quality. It won't sound as clear as a digital recording, but should not sound scratchy or warbled.
Find as many records as you can in the same Beats Per Minute (BPM) range. A DJ needs several records since he usually doesn't play an entire song. Dig through the crates or aisles to find record that you can use a spinet of. Listen to the beginnings, endings, and break sections of records.
Negotiate with the record storeowner if possible. If you're buying in batches you might be able to get a good deal. Record storeowners can also be knowledgeable about the different genres of music. Once you have found something you like in a record store, you can often get the owner to order vinyl records for you, direct from the record label.