Barca have won just one of their last four league games, dealing a blow to their hopes of defending their LaLiga title and leaving them third in the standings, nine points behind leaders Girona and seven behind rivals Real Madrid.
The Catalan outfit were held to a 1-1 draw at Valencia in their last league match, in which striker Robert Lewandowski missed a slew of chances, before Valencia's Hugo Guillamon scored a 70th-minute equaliser.
"We lack conviction in all areas and need to be more effective and decisive in defence and attack," Xavi told reporters ahead of Wednesday's game at the Estadi Olimpic Lluis Companys against bottom club Almeria.
"I think the reason why we are nine points behind Girona is because of our own mistakes... It's a difference that we can make up. It's a matter of not slipping up and starting to rack up wins, starting against Almeria.
"To win the league, we need to take things step by step, we need to win games, we need the fans and we need to put in a complete performance, which is something we haven't been doing yet."
Barcelona have also had to contend with a string of injuries to key players, with keeper Marc-Andre ter Stegen having undergone back surgery and Spain midfielder Gavi sidelined with a ligament tear.
Xavi said Barca are hoping to sign new players in the January transfer window, but added that they would spend within their means after having had financial struggles in recent years.
"What we are looking for is to always strengthen the squad, in every window if we can," the Spaniard said.
"With Gavi's injury we need a profile of his type but it's not easy to find another Gavi on the market. We will see the famous (financial) 'fair play'. If we have space, we will sign someone. We are working with Deco (sporting director) and the president."
Xavi added that Brazilian forward Vitor Roque, signed from Club Athletico Paranaense, would join the squad after the LaLiga's winter break.
