When you place a trade, you are either buying or selling a security in the market. Buyers who are buying believe  the security value will likely rise. Sellers ( or Bears ) generally think its value is going to fall. 

Buyers and sellers affect the supply and demand of the security and therefore its price. A long position in trading is when you buy a security and short position is when you sell it. When buyers outweigh demand increase and price of the asset rises. When sellers outweigh buyers, supply increases and demand and price drops.

Traders / Investors typically will use a broker account to purchase securities. 

Buying-And-Selling Vs Security Price Impact

Practice Buying and Selling – Beginner